Lily wasn't oblivious to how Snape felt, she just didn't feel the same way about him that he did about her. She married James..after he'd stopped acting like such a prick.
He lived a miserable life because of his own choices. He became a Death Eater, and I think it is safe to say that there probably wasn't a single Death Eater who never at one point got blood on their hands. Even if he never did(which I doubt) he still stood by as innocents were tortured and murdered by his friends.
Then when he finds out what Voldemort plans to do involving Lily and Harry..the first thing he does is ask if he would spare Lily and only kill the child. That's not a good person and I am sorry, but 17 years isn't enough to make up for all the horrible things whilst loyal to Voldemort. It's not like he was in prison, and really for most of those 17 years Voldemort wasn't around. Snape was only really a double agent for like..2-3 years. Nope, that isn't enough. Do we in our society only give killers 17 years in jail?
__________________ "I know it's gonna work because it's impossible"-George Lucas