In the previous mitzvah, we discussed how a slave who runs to Israel from a foreign land may not be returned to his master. In this mitzvah, we see that we may not oppress the runaway slave, neither verbally nor physically.
The reason for this mitzvah is similar to the prohibition against oppressing a convert (Mitzvah #63). People are people and you’re no better than the runaway slave. Furthermore, he’s out of his element and lacking a support system, so it’s especially reprehensible to kick him when he’s down.
This mitzvah applies at all times. It is discussed in the Talmud in tractate Gittin (45a) and is codified in the Shulchan Aruch in Yoreh Deah 267. This mitzvah is #255 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos; it is not listed in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.