I'd be the first one making the argument you're making if the source material was something I didn't give a shit about (see: NOAH). But since I do, I can't remove myself from it. Just doesn't sit well with me personally, and for people who felt that way about MoS, it's impossible to not think about that scene when the similar set-up transpires in Cap 2.

(I also think there's another argument to be made that the killing in MoS would've worked for me if I felt they'd emotionally earned it through the development of the character of Clark in the film. I felt it wasn't emotionally earned. In the Frank Miller Batman, all that dark shit feels earned. That's probably the "remove all biases" argument, but I might as well keep it 100% and not try to deny that "Superman doesn't kill" hasn't been ingrained in me since birth.)