Basically, thieves are forced to play as Neutral or Evil characters.
Thief. You get this when stealing from a Good or Neutral player. It lasts for 24 real-world hours, and prevents you from choosing a Good Alignment.
If a thief chooses to be neutral, you can kill him, but you'll be flagged as Aggressive.
Hurting a Good or Neutral player doesn’t turn your Alignment to Evil, but flags you as Aggressive – a condition that gets you treated as Evil in many ways. For instance, Good players become able to hurt you, while other Neutral players become able to do so as well without consequences.
If he for some reason chooses to be evil, you can kill him without any penalties.
While Evil, you’re free to perform hostile actions against any other player – just like when you’re Neutral. However, other players can also do so against you, without consequences – such as being flagged as Thief or Murderer or losing Karma. In fact, they will gain Karma by getting the world rid of you!
So basically, if you see an evil thief, you can kill him with no consequences. If you see a neutral one, you can kill him, but you'll be flagged as Aggressive. If you want to kill someone who isn't a Murderer and isn't Evil, there will be consequences.