Thieves are mostly uneducated people/aliens who wandered in the street in their childhood.
Mostly a Thief would be an orphan, the common reason that might drive him into doing acts of thievery.
There's another way for a Thief to pick this role, as he has brilliant criminal mind and thirst for exciement, and in other words, "Easy cash".
Now the problem about being a thief is that you can't actually steal from others, unless it's a RP.
The FCRS system, developed by Cindy Kellner, would probably insert an 'Ingame' cash, means you won't be spending lindens, but other kind of money which is attached to the system.
Until the system will be released, and there will be an option for thieves to pickpocket people for real, the Thief subject is closed.
Meanwhile, you can play a Spy, or an Assassin role.