You can't kill Victor in the beginning, but you can later. But I think that the character you really can't kill is Lanius, because you can't kill him until the very very end of the game. You don't see him until that time, but that's the whole point. He's Caesar's second-in-command and if you did have a chance to kill him before the end, they'd have to come up with a third-in-command, then a fourth-in-command, and so on and so on, and make ending slides for each of them.