Personally I don't see the appeal of making Superman evil or dark. What makes him a compelling character to me is that he has the strength to resist the temptation that comes with his power. People say villains are more interesting than heroes, but I think villains are simpler because they just succumb to their temptations and flaws, while heroes are more complex because they resist those temptations and flaws. Superman's power is so vast that his restraint and self-discipline have to be equally vast, and I find that tension compelling. Some of the best explorations of Superman's psyche are about that need to maintain control -- the "World of Cardboard" speech in Justice League Unlimited, the "Mine Games" episode of Superboy where he revealed his greatest wish was to be normal and have "the freedom to compromise," this week's Superman & Lois episode where he talked to his son about how the people's trust in him is tested every time he uses his powers. What makes Superman so admirable and fascinating is that he wages that battle every day and wins it -- because he has to win it, since the alternative is unthinkable. He's Atlas carrying the weight of the world on his shoulders, except the force he's holding in check is his own.
So stories where Superman fails in that control and just starts killing people... to me, those take away the thing that makes him Superman, that makes him compelling.
