I played some Mortal Online, a real joy ... here I was practising swinging my blade at a tree ( I understood merely swinging would improve your character use of the weapon ) and a fellow gamer deliberately stood in front of my swing, took a hit and used the ultimate spell in the game... he typed 'guards', then looted my corpse after the gaurdians of justice had beaten me to a pulp. True skill that.
It happens in every game where you give the player too much freedom, Mortal Online I got ganked so often it took me 2 weeks to find a tree to chop down wood, in Age of Conan the players would wait at the spawning point of an area and gank the incoming player before he had actually loaded into the world... he would be dead before he could see the action.
Players wouldnt just act like thieves and bandits in the game world, they would abuse and exploit every glitch possible, hack the game if they could do so safely and pick on new players because its easy... they aint playing the game, they are having fun by purely ruining the game for others, I mean why kill a new player that doesnt have anything of value to take ?
In EvE they have a competition called Hulkageddon... the aim of the game is for players to attack and destroy players that are mining, not to steal the ore or materials but purely as a score tally, x amount of z mining class ship... the ones who destroyed the most was the winner, hell there was even prizes.
That was a lot of fun for the miners you could imagine, as a Hulk costed a lot in ISK plus took a relatively long time to skill up to use... co-incidently the ones who organised the event used to sell many Hulks, and the price of the Hulks shot up.
The true problem is to allow freedom to play as a evil character, but not be a griefer and that is the hard part.