People only hold their morals when their around their piers. That's why they say you never really know anyone unless you watch behind closed doors. They have people married to serial killers for years and they never even suspect a thing. (A bit extreme but I think it's a good example of not knowing people's true intentions and letting trust blind you)
Serial killers are generally regarded to have a mental defect. They don't operate in the way a normal person does. Its true you can never really know a person but given that most people don't kill people and wear their skin like a coat I don't feel that argument holds water. Also trust has no bearing on the discussion.
People don't always need peers to act in a "moral" way. Nature plays into the choices you make but ultimately who you are guides behavior. The choice to act a certain way is made long before the situation arises. Simply put there are things you just won't do, or will do simply because you are you. Money, social pressure, etc can be factors but they are not necessarily the controlling ones.
I've read social experiments where people give people large sums of money to hold for them or put a wallet on the ground with money sticking out and people act in a moral way. Even though in these cases there is no reason for anyone to think that in taking the money that the person they'd be stealing from could track them down. Nor would anyone that sees them be able to call them on stealing since no observer would know they'd be stealing.
Society has a code of law/morals that are instilled in people from a young age. Your behavior is shaped over time to program you to act in a certain way. Even the realization that you've been programmed can't free you from it. Going back to your serial killer, the defect most have is a lack of emotion. These exist outside of society and are generally regarded as part of the human condition. Empathy and pity can guide behavior as much as anger and envy. Most people don't feel good about hurting others, either during t contemplation, the act, or after the fact. So its entirely possibly tha even without a society to inform your behavior you'd still naturally act in a moral way. Humans have a great capacity for evil, but they also have an equal capacity for good. So far it appears we're neutral at the beginning and learn to favor one or the other. An by your logic the social nature of humans would likely lead to more good than evil.
