No ( to the killing ques), yes (to the nobody's perfect statement), yes ( to everybody makes mistakes)....and that's a darn good question( to why should people believe in sin if they don't believe in God)
I don't know if I can answer that. All I can say is that the evidences of sin are there...ie. terrorists....lying.... both lead to pain.
If one does not believe in God, then why should they strive to be "good"....I think I'd better stop right here.....