That\'s a valid point. But, having role-played assassins (notice I said \"roleplayed\" and not just random killers) would add to the game.
What you could do is limit how and where you could learn the skills necessary to become an assassin (picking locks, maybe stalking, and certainly backstabbing or some form thereof). Thieves\' and assassins\' guilds aren\'t exactly easy to find, after all. You have to know who to talk to, and even then they\'ll probably set you a few hard tasks before they agree to teach you anything (quests, anyone?).
and then walk around happily killing everyone without the guards doing anything.
True... but think about it. Your average Joe on the street
doesn\'t carry much money, nor is he going to be worth many XP.
If there is an assassins\' guild (or some group that teaches these skills) they aren\'t going to appreciate someone killing off innocent people in \"their\" territory, without their consent. The Watch may not do anything about it, but the assassins\' guild certainly will. A few muggings is fine; such things happen all the time, and aren\'t really of concern to the guild. Even a few back-alley stabbings are acceptable. But someone who consistently breaks into homes to rob and/or kill people without guild consent will quickly be put under sanction, including but not limited to: refusal to train any further skills; refusal to use the guild headquarters (if there is one); or even being killed, run out of town, or turned in to the City Watch. The Watch doesn\'t see everything, but the guilds do, and they\'ll know if you go on a killing spree.