although it's a popular idea, i do not agree with this notion that every person is partially good and partially evil, and that it all depends upon the circumstances. although i do agree that circumstances can bring out aggression or defensive behavior, i don't consider it evil or cruel to try to survive.
some people experience pleasure in causing another's pain; these people are sadists, and i consider that evil. other people (i hope the majority!) when coerced often conform to what is expected, and allow abuse to go on, but they don't delight in it themselves. i wouldn't call that evil, but self-preservation. like when you looked away at someone being restrained because staff said to "Pay attention to the person talking!"
didn't we all see this range of behavior at straight?
linchpin, you're right that you can't trust everyone, but you can trust some. it's just hard to differentiate who is trustworthy and who is not. you can probably trust your girlfriend.