You are right suggesting the universe is a computer simulation no more scientific than believing there is a god (unless you have a way to prove it). There is nothing wrong with thinking about alternatives. The problem comes when you base your actions that cannot prove or disprove, and force other people to believe your speculation. Or state it as the "gospel truth" when it is just speculation.
It may well be that I am imagining everybody else, but the moment I start killing other people because they are not really real it becomes a problem.