Why not be anonymously nice?
For the same reason why we sh*it in restrooms and eat in restaurants. Same place, same person - different acts.
Internet is where we sh*it - a function that is opposite to dining but equally pleasant and actually necessary to keep things humming. This place is a restroom and a place where we purge.
Also, to know what love is, we need to practice hate. If you can't hate, you are damaged goods with a malfunctioning instinct of self-preservation.
In fact, when A says to B "I hate you", A cares about B. A marriage is done when both are politely silent, not when they scream and swear.