It's hard to argue the world wouldn't be a better place, but I think it's hard to know where to draw the line. It's all fine until you're in the minority, then I can't imagine you'd want people killing someone you like and respect even if they're nearly universally despised. It's a slippery slope probably best avoided.

Those are the murders that the police frequently don't expend a lot of effort in investigating.

There's actually a lot of murders that never get solved.