Eve
1. Yesno. Eve's never killed anyone directly. For years, intel she gathered was used to help assassins like Steven do their jobs, and last summer she helped Pella go around NYC executing baddies who'd escaped from Ryker's. She did everything but pull the trigger on them. She has mixed feelings about this.
2. Defense of self or others as a last resort. You should do absolutely everything else you can to avoid killing them, even if it puts you at risk. But putting innocents, especially children, at risk is different. Your conscience can't be the reason you let other people die. Which is also why Eve was willing to help Pella; she knew those people were going to die, even if she had nothing to do with it, but she reasoned that with her help, she could minimize the risk to others.
I'm really not sure what would happen to Eve if she killed. I think she's the one of mine most likely to - she's definitely the one who's come closest.