I would hesitate to suggest a causal link between believing, or considering, Simulation theory as necessarily being the domain of lonely people to justify murder. How many murders occurred because of a belief in God? How many people believe that violence in movies and video games promotes violence? Statistically, it doesn't bear out.

There isn't anything inherently wrong with the theory, other than there is no way to validate or falsify the theory. It's like a theory of God. You can only take rational discourse so far.

I think it is reasonable to assume we're not in a simulation. I don't think it can be ruled out absolutely, in terms of an intellectual exercise. I suspect, though, even if it is a simulation, ethical rules for playing a simulation still apply. In fact, if we all agreed to be here, being ethical may be even more important than if it's not a game.