A person who chooses to hate another person on the internet for no apparent reason, or if he has a reason, it is often a half-assed one. Often the person is a victim of multiple personality disorder, schizophrenia, or other diseases that cause him to make up his own reality, often including several made-up personalities who praise his own work, attack or ignore those who criticize his work, and attack others' work. Sometimes there may also be a case where he accuses others of theft, when he himself is a thief.
Zack had MPD, which caused him to become delusional that Mike was being mean to him. So out of revenge, he and his imaginary friends attacked Mike over the internet for no apparent reason. He is a perfect example of a hater.