Thomas Shey
Legend
RPGs are group games. They're cooperative. Having a player decide their character is going to attack, steal from, or otherwise harm the other members of the group goes against the very nature of the game because they are not being cooperative.
I have to point out a game can be group oriented and still competitive. I've seen any number of RPGs that included some degree of inter-player conflict over the years, right from back at the start of the hobby. Sometimes its okay (when kept to an appropriate level and when people can separate character from player issues) sometimes its not, but its not intrinsically counter to how RPGs work (and in fact, some of them--Amber Diceless Roleplaying comes to mind) assume a certain amount of it right from the get go. Its an issue of group style and convention, not a law of the hobby.