Why not?
I don't want to run a game for murder-hoboes, so I don't. Other groups should do what they have fun with.
In a might-makes-right society, which many D&D settings strongly tend toward, being or becoming the biggest baddest band o' murderhoboes in the land would seem to be a highly viable career goal.
True, but I don't think PCs are unique and there's no reason to believe they could automatically just do whatever they want. Maybe they could, maybe they couldn't. But piss off enough people and you may well be facing a small army (or a small army of assassins) and everybody has to sleep sometime. If the characters have managed to organize a small army themselves? That's a different story of course. Just not one I want to deal with.
That, and for my part I see D&D settings as being every bit as violent as the historical real world, if not sometimes more so.
Depending on the scenario D&D world could easily be more violent. After all, we don't have ghosts and beasties running around, no invading fiendish army or zombie hordes.