This was my reply to your pm you are wanting an answer soon and you haven't read it so I'll post it here.

If their only other option is being put to sleep couldn't you at least give them a chance? Picture being alone and scared in a strange place and somebody you don't know (and possibly you associate with being hurt, as some person on the street may have abused them) lifts you up...you are terrified so you react. Well, if you are a dog that reaction happens to be a bite. It will take time to realize that all humans are not bad.

I think speaking with a behaviorist like Veegan said is a good idea.