Modesty and fear are the main factors that make us behave the way we do. We control our expressions of anger, hatred, disgust, lust etc. because of them. Anonymity provides the opportunity to be free from the need to be modest or feared to express their true opinions. It also shows the true character of a person. Most of the anonymous comments we see in blogs are expressions of such depressed opinions.
When the anonymity of those people is lost, they get hyper defensive of their acts as they know in the back of their minds that their true tarnished personalities are being exposed.
People who present themselves with screen names are often just being defensive of their opinions than being guilty conscious about it. They represent their personality with grace unlike the anonymous. These people wouldn’t mind exposing their true identities to those they trust.