A son respects his father and a servant respects his master, so why don't the Jews give G-d proper respect? The kohanim (priests) are negligent when it comes to their responsibility to educate the people. And how have they disrespected G-d? They offered sacrifices that were sick and blind. They would never give a human ruler such an animal as a gift, so why do they disrespect G-d with such offerings? Do they think He'll heed their prayers when they treat Him so lightly? If only there were one person who would take a stand! G-d doesn't want their half-hearted service.
Even the other nations recognize that there is a Creator over their idols, but the Jews disgrace Him by saying that the sacrifices of His altar are insignificant. G-d curses those who would justify and make excuses, offering a lesser animal as a sacrifice rather than the best of his flocks. G-d is the Great King; if the other nations recognize this, how much more should the Jews!
Through Malachi, G-d charges the kohanim (priests) with accepting unfit sacrifices. If they do not listen, He will curse what otherwise would be blessed. In fact, He has already begun to do so! G-d will stop the grain from growing and G-d will toss the manure of their sacrifices in their faces. G-d reminds the kohanim how He forged a covenant specifically with the Tribe of Levi, to administer to Him in the Temple. Pinchas, who was zealous for G-d, earned this covenant, which is called one of peace, for himself and his descendants. The kohanim should be pure, and they should be like representatives of G-d on Earth.