I understand what people mean when they say that the game Vampire "brings out the worst in people". I just don't necessarily agree.
Again, I think it depends on the perception of whether or not what that person does is okay or not. "That vampire straight up murdered my vampire for the smallest slight. That person is the worst." Did Vampire bring out the worst in that person? Or is that just what one person thought was a completely acceptable response in the bounds of that game world?
Theme is a big deal. But there can be a big difference between a game and a theme on the game. If you throw an established IP like Vampire onto a MU and do not clearly establish your own theme, you leave it up to everyone to join with their own interpretation of it and thus their own standards of acceptable behavior informed on what they have or have not read (because we all know that much of the theme are in parts of the books that many people skip in favor of studying the dice and power rules).
So I throw support behind what various people have said in different forms which is that it is up to staff to maintain the kind of theme and community (game) they want on their MU. And when staff does that it works. And when they don't, it is chaos where some people love the game and others are collateral damage for those first people. Sometimes it just depends on whether you were able to get "in" with the right people, i.e. "cliques".
P.S. I do believe there are many people who can separate character and player, but it is generally safer to assume that they can't until you know that player well enough to be sure that they can. But even then, sometimes you turn out to have just judged them wrong. So is it possible, yes. Is it likely, probably not.