I think this aligns with the common and infuriating Western media trope where the hero can kill any number of low level unidentified grunts but as soon as they come to the big boss guy they suddenly get morals and have to let him live.

All the pain and suffering caused by Gaddafi happened off screen to unidentified people so we were silent. His killing happened very much on screen to a recognizable "bad guy" so suddenly we found moral outrage.