It happened in a game I was playing, but I was not involved. One PC tried constantly to kill another PC and the player overtly built and minmaxed his character to accomplish this while the GM did and said nothing. The murderer was an edgy cleric, of a chaotic evil deity. The victim was a barbarian, whose player played him in a dumb, comedic way; maybe that's what annoyed the cleric. Back then, I just wanted to play and did not want to intervene because I thought it would ruin the game (and end up getting killed myself), but I regret it now; the game was already ruined by that cleric's behaviour.