This isn't true at all.
If you're consistent with your roleplay and don't take things too far, it is well within the rights of a player to make sub-optimal decisions for the sake of playing a character with odd quirks and flaws.
There are a million things that could upset other players that are perfectly legitimate forms of "its what my character would do".
Saying "its what my character would do" is the whole fucking point of D&D. We aren't RPing ourselves. We're literally playing a fantasy game with magic and gods and the like.
Are you telling me that our PC's need to be more logical than real people when they make decisions?
Just think of how many actual people do insane shit, get themselves or others killed due to stupidity, angry outbursts, etc.
It is a lot. And if you're roleplaying a fantasy creature with flaws, desires, prejudices, and motivations, at some point those are going to conflict with the flaws, desires, prejudices, and motivations of others at the table. And when that happens, everyone at the table needs to be mature enough to say, "That's what my character would do."
Up to and including PvP combat.
But that being said, your desires, motivations, prejudices, etc. need to be well established and within the margins of acceptable table manners. If you're just busting out some deep-seated hatred of X 5 sessions in that hasn't been mentioned at all outside of your character sheet, its bullshit. Or if you have some creepy fantasy/fetish that you want to RP out, its bullshit.
But there is absolutely nothing wrong with players keeping their PC's bound to their intended limitations/parameters, even if it means they or other PC's might die because of it.
And like I said before, if everyone is on the same page about RP and they take it all in a mature way, two PC's may conflict and both 'do what their characters would do', including a fight to the death, without anyone getting butthurt.
Unfortunately, not everyone has this understandi character's motivations/prejudices to the party in a clear fashion (or often enough) and "what my character would do" sounds really similar to 'I just came up with this character trait because I was bored.'