They saw him [Yosef] from a distance, and before he approached them they were plotting against him to kill him. .. Reuven heard and rescued him from their hands. He said, "Let us not kill him." .. His purpose was to rescue him from their hands, to bring him back to his father.
Let's dig a bit. Our Torah testifies that Reuven's ultimate goal is to save Yosef entirely. Reuven's tall task however is to convince his brothers to spare Yosef while also validating their position. Remember while we know Reuven's ultimate agenda, his brothers do not; therefore he must argue on their terms. Reuven offers a curious compromise.
Reuven said to them, "Do not commit bloodshed. Throw him into this wilderness pit .. and do not lay a hand on him."
Ostensibly, Reuven's argument is that Yosef is culpable, but it is simply inappropriate for us to kill him. We must then wonder: is not putting Yosef in a pit, away from sustenance and salvation, tantamount to the same thing? Facing this very question, Raman concludes that Reuven's argument is a technical halachic one:
For also I hate him and want him to be killed by others, but you shall not be spillers of blood with your own hands - God forbid
Halacha distinguishes between direct and grama - indirect causation. The former is punishable in beit din and the latter in the heavenly court. Thus Reuven argues to his brothers that he agrees in principle to the act, but he seeks to mitigate the damage.
A famous Talmudic midrash [Shabbat 22a- cited by Rashi] however imperils this argument
From the statement: "The pit was empty" don't I know that there was no water in it? .. [It teaches] there was no water in it but there were snakes and scorpions in it.
If there were snakes and scorpions in the pit, it would surely constitute this-worldly murder!? Witness the following Talmudic passage: [Sanhedrin 77a]
Raba said: If one bound his neighbour and he died of starvation, he is not liable to execution. Raba also said: If he bound him in the sun, and he died, or in a place of intense cold and he died, he is liable; but if the sun was yet to appear, or the cold to make itself felt, he is not.
As long as the agent of death is present and there is no way out, a bona fide murder it is! Thus, Ramban's approach and Rashi's midrash seem at odds.
For Rashi's midrash, we need a new approach to Reuven's gambit. We turn to the incredibly deep words of the Zohar, which makes a brilliant and hashkafically (philosophically) challenging claim. The Zohar's logic is picked up by Ohr HaChaim and Alshich
R. Isaac said: 'Seeing that the pit contained serpents and scorpions, how could Reuven have advised that Joseph should be cast into it in order that "he might deliver him out of their hand, to restore him to his father"? Had he no fear of the serpents and scorpions attacking Joseph .. But the truth is that Reuben perceived the intense enmity of the brethren towards Joseph .. and he therefore thought that it was better for him to fall into the pit of serpents and scorpions than to be delivered into the hands of enemies who would have no mercy on him. Hence the saying: "Rather should a man throw himself into a fire or a pit full of serpents and scorpions, than be delivered into the hands of his enemies."
For the Zohar then, Reuven's argument to his brothers is not a halachic one, but a deep philosophical one - and it comes from a place of introspection and self doubt. To properly appreciate its notion, let us step back in time a moment .. Yaakov sends Yosef ; when he finally catches up to his brothers, who spot him:
One man said to another, "Here comes the dreamer. Now, come let us kill him and throw him into one of the pits; and we will say that a wild beast devoured him. Then we will see what will become of his dreams."
If Yosef is to die, then what is there to see about the dreams? Thus Rashi quotes the midrash
The Divine Spirit is saying this: They are saying, "Let us kill him," but, the verse concludes "Let us see what will become of his dreams," [i.e.,] let us see whose words will be fulfilled, yours or Mine? It is impossible that they [the brothers] said, "Let us see what will become of his dreams," for if they kill him, his dreams would [automatically] have no meaning.
The cynic does not accept our question, for the brother's comments may be understood to reflect deep sarcasm; the brothers' in effect are saying we will do it our way - now let us see what will be with his dreams. Indeed, this is one approach advocated by Ramban. So, either we are dealing with a prophetic spirit or a sarcastic sibling. There is yet a 3rd way - hinted to by Ramban and explicated by Ohr Hachaim.
A simple question to ponder: Were the brothers sure? Yes, they acted to eliminate Yosef, whom they defined as a dangerous pursuer who sought to out them from Klal Yisrael [cf. Seforno] - but did they ever harbor doubts? Did they even allow for the notion that Yosef was not merely a megalomaniac par excellence seeking absolute power but maybe something much deeper was going on here - maybe o' maybe those dreams were even prophecy? 
A scintillating inference of Ohr Hachaim demonstrates the latter. Yosef dreams of their sheaves bowing to his. The brothers hate him. Then he dreams of 11 stars, the sun and the moon bowing down to him. Here the Torah records their emotions with a slight difference.
His brothers were jealous of him and his father watched the matter
They move from hatred to jealousy. Hatred is equal opportunity, but jealousy bespeaks belief in the other. I am jealous of the other when there is something there that I consider significant! For Yosef to speak of lording over his parents transcends simple ego - it is not simply narcissistic, it is incomprehensible and not normal - so strange that perhaps it is prophetic!
Now Yosef comes to greet them. They mock the dreamer and plot to get rid of Yosef - but that nagging doubt persists: maybe he is a true tzadik. They comfort themselves: Halachically, we consider Yosef a pursuer, but we know that if Yosef is real - then we will be powerless and he will somehow escape. In other words, the dreams will prevail, for we have no ability to harm him against Divine will. In the midst of all the craziness, the brothers' introduce self-doubt and hashgacha [Divine providence] into their calculations, finally reassuring themselves that their actions are ultimately meaningless. Here Reuven pounces, making a remarkable statement:
No! he argues on their terms. God has endowed man with such free will that man can tinker with the Divine plan. You may indeed be killing Yosef the tzadik. Animals however do not have free will. Put Yosef into the pit with animals - for that is the ultimate litmus test of his righteousness.
Here the thinking Jew must wonder, how can it possibly be that one man can snuff the other's life out simply because he has free will; can we play dice with God's universe? It is a huge topic - far beyond the confines of our discussion. Sources are adduced back and forth. Many reject Ohr Hachaim's understanding of this Zohar completely. Indeed, Ramban himself in several places rejects this notion. For him, it is absolutely inconceivable that either animal or man can hold sway over the Divine plan. For Ramban and others, Reuven's logic must be technical - halachic.
Netziv however qualifies the Zohar. Human free will can push the Divine envelope only when there is potential culpability. A purely righteous person will never be subject to another man's free will. A somewhat righteous person is under the influence of another’s free will, akin to a she'as hasakanah, a dangerous situation that evokes intense Heavenly scrutiny on an individual.
For Netziv then, Reuven's argument to his brothers goes as follows: You allow that Yosef may be righteous, but he certainly is not without sin. Had he been purely righteous, you would never have control over him. Do you want to take that chance? Let us throw him to the animals, to those who have no free will and "allow" God to determine his fate. The brothers' agree and Reuven has bought precious time.
What moves me here is that even the brothers who must have been quite passionate and firm in their convictions to engage Yosef in the manner they did, are ultimately able to admit to self-doubt.
To all of us who remain firmly entrenched in our places of righteous indignation or solidly stuck in our absolutist attitudes, we might consider the bracha of self-doubt - one which leads to significant introspection - for within it lays a path to teshuva, a road that the brothers ultimately walk on.
1. Two possible reconciliations: The brothers did not know about the snakes and the scorpions - perhaps the pit was too deep. This is the famous position of Torah Temimah, who creatively explains the Talmudic juxtaposition between Chanukah in this manner. A second salvage approach of Chizkuni is that while the brothers originally placed him in an empty pit, when Reuven left the scene they re-placed him in a different snake-filled pit.
2. Indeed Ramban explains that Yosef did not contact his father Egypt precisely because he believed that the dreams were prophecy as of yet unfulfilled. To contact Yaakov would have been tantamount to violating the prophetic vision. Also, Rabbein Bechayei explains that Yosef was compelled to relate his dreams to his brothers, else he would have been culpable as a prophet who squelches his prophecy.
3. Cf. Ramban Bereishis 15 where he explains why Egyptians are punished for simply executing the Divine will of oppressing the Jews. In one explanation he teaches that while they were doing the Divine will, that was not their intention. Thus Reuven who kills Shimon is culpable even as the Divine plan is for Shimon to die! A similar notion is presented by Sefer HaIkarim in dissecting Hevel's death. There he wonders why Hevel deserved to die and he posits what Hevel's sin was. Cf. however, Michtav M'eliyahu [vol. 4] who questions this notion on the basis of the midrash that has Hashem chided Kayin for the blood of Havel and his descendants are crying out from the ground. If Hevel is supposed to die, then how can there be unborn descendants?
4. Alshich thus explains Megillah 11a that states that the Purim salvation was greater than Neuvachednezzar and Paroh – for the latter was in the hands of animals as opposed to Haman that was in the hands of man.
5. He further explains that Yaakov, who is sure of Yosef's absolute righteousness is able to send Yosef and put him in the brothers' grasp - because it is inconceivable that they would have power over him!