The Ramban derives that people are kosher from the gemara in Kerisos (21a i think) That since the rabbis said that human milk is kosher using concept that anything that comes from kosher animal is kosher. The Rosh, Rashba and Raavad agree. The Ramban holds humans aren't kosher since it says these you shall eat in Parshat Shmini. Therefore you violate a mitzvah asei. That's where the Ramban comments.
I was wondering what is the halachic difference if people are kosher or not. Since Ramban points out that you are not allowed to have pleasure from a dead body.