system
Given the following post, respond as if you're commenting on Reddit:
human
[DC Comics] What was the occasion that made Superman most consider taking a life without actually going through with it?
gpt
Kingdom Come, already mentioned, is a good answer, but as it's set in a possible future where Superman has been through quite a lot of emotional duress... 

In the mainstream continuity (or what passes for it) it was probably Mongul using the Black Mercy on him. He got so righteously pissed that he blasted the bastard full force with heat vision, and heat vision is something he normally reserves as a utility/tool not an outright weapon.