You can run over non-player characters in a video game with total impunity. You can stab and shoot and blow up holodeck characters to your heart’s content. You can beat up and blast anyone you want in the matrix, so what motivates you to treat your fellow dreamstate characters any differently? Why are you so well behaved?
If you can act without consequence, then you have absolute power, and absolute power corrupts absolutely. You might not start vivisecting nuns on day one, but you’ll get there eventually because boredom will insist. Why shouldn’t you go on a workplace shooting spree tomorrow? All you have to lose is your life, which is not yours in the first place. Our disbelief is suspended, we believe this is real, that it matters what we do. Rationally, there’s no reason not to obey every terrible impulse, but irrationally, we don’t want to pay the price.