I have been debating this a bit with some of my friends and I have come to the conclusion that one player using his agency to remove another player's character from the game needs to be completely about what is going on in the game world, and the other player needs to be in on it. I also don't like to have competition as an element in TTRPGs. I don't want there to be a final win/lose condition, don't want players competing against the GM or vice versa, or each other. When someone has an awesome character you should see that as being a great addition to your team, not a threat to your ego. Also a powerful character should not be a wedge to be used against another player unless it is all about what is actually going on in the game world--not about what is happening between the players around the table.
So many games, over the years, where someone passes a note to the GM and then the next thing you know two PC's are rolling initiative against each other. I feel like one of the reasons this happens is that there is the assumption that I can weaponize my agency against yours. Even in games with tyrannical control-freak GM's it seems like once the characters decide to fight, the GM often abdicates responsibility and becomes caeser watching gladiators fight for the amusement.
How do you handle this in your games? Are players advised as to what it would look like if a fight breaks out between PCs? Are they advised as to what conditions this is allowed to occur?
I think this should be a point of clarification at the game start. Players should be able to know the limits of their agency where removing another player's character from play is concerned.