You never really know a person by who they are online... But in reality, you can never tell in person, either. We think we know what a person is like by simply gauging their opinions and reactions. But some people are very good at deception, and some of us are quite easily led. Even the safest of assumptions could turn out to be wrong.
A person might seem sincere - go out of their way to help people, even - and then you find that it was either all an act or they're not quite as nice as they've made themselves out to be.
Everyone has character flaws. It doesn't make them necessarily bad. We sometimes don't take into consideration that our actions actually have a positive or negative effect on others. It's quite often something that's very easily overlooked. So when someone is wronged by our actions, it can come as a complete surprise.
We all need to put the "shoe on the other foot" once in awhile...