You bruise, but you don't kill, do you . . . Clark? - Batman, Justice League: War
Everybody knows Superman is the Big Blue Boy Scout. Sure, he and Brainiac might level half of Metropolis while duking it out. In the end, though, he'll find a way to banish the villain without destroying him. Superman doesn't kill his enemies, except when he absolutely must, and even then it's a shocking and traumatic experience. Witness, for example, his reaction to killing Zod in Man of Steel, or even accidentally causing Doctor Light's death in last year's "Trinity War" story arc.1
However, it wasn't always that way. Supes began his career as a bruiser, right from Superman #1 in 1939. In one story in that magazine, he kills a military torturer by flinging him over the horizon, then causes the death of an enemy pilot by wrecking his plane in midair.2 The body count just goes up from there.