"Good' and "evil"are subjective terms. Just like certain countries have strict laws punishing people for theft by cutting off a hand while other countries simply put thieves in prison; what good and bad are is relative.
Mental illness can also play a part. Some prison inmates have a psychological disorder of one form or another. Legally speaking they are not "insane" but they may not have much control over their anger or behavior. At times this may make them unable to objectively judge their actions as "evil" or anti-Social.