I think some people do it because being good is 'too mainstream' or something like that. Honestly, I don't join those roleplays anyway. They are rather shallow.
Purely good people and purely evil people are rare, a battle with two sides is realistically way more complicated than "you're evil, I have to kill you!" A Good Vs. Evil roleplay has no really interesting aspects and quickly becomes dull.
And demons and angels...are so overdone. :/
