In single player, I won't lie, I generally play for the evil faction first if that's an option.
However, in online games, I want to be a good person at least to people in my faction and even then I only attack people if they attack me.

Lets take for example...DCUO.

Speaking about the PVP server, here.

Just a bit of background info. DCUO is an MMORPG where you play as a superhero or super villain and you can customise what kind of power you get and what kind of weapon you wield. In the PVP server, Heroes and Villains can fight if they run across eachother in the two large maps.

Anyways...

I remember sometime from my time in the PVP server.
Level 30 people camping newbies at their quest sites. (Level 30 is the highest level in game.)
They just stand there waiting for newbies to come in or come out and kill them. Why? Why not go find some other level thirties and kill them? Are you that shitty at the game and that much of a prick to wait for level threes and fours, people who barely finished the tutorial, to come into their first real quest and kill them?

There were some cool people who when they saw a newbie, rather than killing them, just followed them around. (I've had this happen to me. A level thirty hero was following me around and I was like wutlol.) and only attacked if the newbie was dumb and attacked them.
Like this one time. 
[hider=Backstory of my Char]I was Minion Zero, a former Minion to Gen Eric the Villanious, who after getting into a freak accident that gave him ice powers and finding a steel rod to use as a staff and killed GETV, became a super villain.[/hider]
I was around level 15-ish and on my way to a quest, I found a newbie hero. I just went up to him and posed, saying something like "HELLO, HERO. HOW ARE YOU THIS FINE DAY?"
I don't know what the heck was going through his head when he saw me but almost the instant he saw me, he hit me for...Barely 13 damage. He got killed in barely four hits. Anyway...

I think the reason he did it was because of the Kill Newbies On Sight people.
It makes newbies kill other people on sight too.

TLDR: Be as assholish as you want in single player.
Be okay on the online multiplayer mode or you'll make other assholes.