“His only wish was to die alone.” “Tough.” is the funniest possible exchange you can have about a guy you just killed.
The fact that you killed him is fine. The problem is you were there while killing him, ruining his chance to die alone. Killing him was pretty much fine.
“Tough” is the correct response because really this ranks somewhere around 137th on the list of most morally dubious things you’ve done. And it’s about to be knocked to 138th by your act of inadvertently setting the apocalypse into motion. Which you kind of just did by killing this guy. Which was fine. But not the apocalypse part.