My own view is that (ignoring simulations for the time being) MWI ideas have no conflict with our usual ethical intuitions and reasonings. Yes, it is the case that when I choose between evil action A and good action B, there will be two branches of the universe - one in which I choose A and one in which I choose B. This will be the case regardless of which choice I make. But this does not make my choice morally insignificant, because I split too, along with the rest of the universe. The version of me that chose evil act A will have to live thereafter with the consequences of that choice. And the version of me that chose B must live with quite different consequences.

What, more than that, could a believer in the moral significance of actions want of his universe?

The situation with respect to simulations is a bit trickier. Suppose I am deciding whether to (A) pull the plug on a simulation which contains millions of sentient (simulated) beings, or (B) allow the simulation to continue. So, I choose, and the universe branches. If I chose A, I must live with the consequences. I don't have that simulation to kick around any more. But, if I were to worry about all the simulated lives that I have so ruthlessly terminated, I can easily reassure myself that I have only terminated a redundant copy of those lives. The (now) master copy of the simulation plays on, over in that parallel universe where I chose B.

Is it wrong to create a simulation and then torture the inhabitants? Well, that is an ethical question, whereas this is a meta-ethical analysis. But the meta-ethical answer to that ethical question is that if you torture simulated beings, then you must live with the consequences of that.