I'd kill for super powers.
On a more serious note, I'd be willing to kill anyone (probably as a last resort, mind you) who poses a significant threat to the well-being my family (at least those people in my family that I'm actually close to), my friends, or people who I would consider neither friends nor family but I like well enough.
There's also situations such as killing a man who is about to kill other people (complete strangers to me).
Something pertinent to this topic that people might want to try out:
Would you kill the fat man?________________
"When we remember we are all mad, the mysteries disappear and life stands explained."
- Samuel Clemens/Mark Twain