Sorry, not buying this.
People aren't responsible for their own behavior? Okay.
A DM could outright tell a player to choose a different race, or just say to the player "if you're hell-bent on doing this, go ahead, but on your head be it - you've been warned".
See, and one of those options is a firm hand on the DM screen, and the other is passive-aggressive bitchcraft. I think it's reasonable to treat one of those courses of action with much more respect than the other.
Also, if you warn the player in question you're going to do something... that's still you choosing to do it, and even if you warned them about it, you're still responsible for being able to justify your own behavior. This isn't some kind of esoteric game theory or progressive ideological screed, it's literally just the basic moral responsibility that should be expected of any functional adult.
You're also assuming the 'immediate murderous bigotry' is going to come from the DM or the setting. IME it far more often comes from the other PCs, either overtly (they shun or attack or even kill it) or covertly (they hang it out to dry at some key moment in a combat and let the enemies kill it).
Well... one... I've never seen that. And two, I don't care because if it's PC versus PC, the players are on even footing and can't just bring in more and more support until they "win".
Three, if they're attacking people who've never made a hostile move toward them, and aren't known to have hurt anyone else, they need to double-check the "alignment" space on their character sheets.