There is no one single answer. Some people may have a warped sense of
humor which may come across as rude. Some may be filled with so much
pain, or have unresolved issues misguiding them to take out their rage
on others. Some may not know any better. Some may regard on-line
interactions as more abstract, or regard words as meaningless noise
especially when directed at others.
Is it fun for those who may deem it such? Perhaps, as once people
falling down and getting hurt used to be "hillarious" at the movies or
in home videos - but would depend on the individual's character.
Finally, are these people that way in real life? Some may be, others
without the protection of online anonymity may like a dog on a leash
become restrained from running as "free and far" as they might
otherwise desire if not "leashed" by the absence of anonymity.