If I had super powers, especially Superman level powers, I wouldn't need to kill. And if you ask me, dozens of years of imprisonment, misery, and then death is much worse than skipping straight to death. Throw someone like Joker in the Phantom Zone where he'll never age and live for eternity until he wants to kill himself; now that's a punishment. The Kyptonians had it right.
If I was a superhero where supervillains existed, of course. I wouldn't have an arch nemesis or a main villain, because after I meet you once, you're dead. Then the world is safer until next time. The whole justifying slippery slope thing means nothing to me. I trust my intelligence and judgment enough that I know I won't go down that road.