It seems strange to me to see a discussion like this conducted in terms of only one principle.

For example, consider how would we know if someone consented to being killed? And how could a consent structure for it be abused? This is where we see it bump against another important principle, that we may not deprive another person of their life without due process of law.

I don't know why you got downvoted, that thing did happen and "consenting adults" was the murderer/cannibal's defense. Another counter-example I'm surprised hasn't been mentioned yet is incest.

"Between consenting adults" is a necessary but unsufficient condition.