[@Sep]
It's not just the killing, it's that S.H.I.E.L.D. is essentially above the law. I mean, I don't see S.H.I.E.L.D. getting permission before launching an operation on a sovereign nation's soil or respecting human rights when dealing with meta-human prisoners. Much like real life espionage agencies, what they do might be necessary but that doesn't make it morally appealing or easy to describe as the actions of a hero.

That was how I was intending to play the character, anyhow. Not as a kill-crazy murderer but as someone who doesn't think twice about killing for the greater good as they see it, a trait that applies to many villains as well.