Well, In Single player RPG's it's just fun to do "Can you help me? " "NO! DIE!" Because mostly you can't reply in your own words.
Still in Online whatever you call it (MMMOOORPORPORPOR or something like that

) Evil isn't defined. in SP RPG it's mostly "if you kill without a very good reason or break the law you're bad"
But in PS the players define what's "evil" to me the concept of "evil" is always a minority who think else the the majority (in general view) Individually viewed it's just the guy that has different idea's about life (not little differences but HUGE differences)
So Evil is when you have other thoughts then the rest (though it's only called evil when the difference is kinda like the complete opposite

)
So i think to RP (in a online game, cause SP is always a bit OOC, still fun but OOC) 'Evil' (in my view of evil) is one who wants to play a character that doesn't necessary have the same thoughts as others

Like a brilliant song says with a strange, almost funny voice "Free thinkers are dangerous"
To clear out my view of Evil:
Think of a sort of hell, everyone accepts it, everyone tortures everyone ect. Then for the majority the 'goodies' are the 'badies'

you see?