Playing with the idea of being evil (doing bad things) can be fun and is essential part of our ability to fantasize. However, at the moment of witnessing suffering (even the virtual one) our empathy kicks in and we demonstrate psychological reaction through strong emotion of wishing to end the suffering of other. So, when you start with your wish to make miserable sims it's in a domain of fantasy, however, when you actually see them suffer, your empathy tells you to do the right thing, and you do it because you can (because at least in a game you are in full control over what happen to others hehe) :)