Best way to have stealing like that is to have it as a pickpocket mechanic that one player preforms on another player, and if the player trying the pickpocket is revealed then the player he was trying to steal from can kill him or if in town the NPC guards swarm the offender and kill him allowing the player to access the corpse and take the head for bounty.
There was player stealing in Ultima Online, not sure if they still have it though, and it added a lot to the game. I can remember one time standing in town with two swords of vanquishing in my pack and suddenly realizing some thief had just stolen them. What happened was I was on my way to the bank to put them there for safe keeping and I got a message on ICQ from a friend so I stopped to answer it and in the 40 seconds to 1 minute that took me the thief pounced and took the swords.
Another time I was going through the gates to enter the city of Trinsic and all of a sudden the trade window pops up and in that window are several of my GM crafted bows with my name stamped on them. A thief had stolen them from me and was giving them back because all he wanted was the thrill of doing the act. Then he spent the next half hour to 45 minutes telling me how to set up my pack with traps on bags within my backpack that would alert me to thieves snooping in my bags...
I miss the feelings things like that brought to the MMO and it really seems the Theme Park MMO's have taken a giant step backwards in terms of what the player is allowed to do in an MMO.
There was also a bounty system in UO and any player who had maybe been ganked by a PK player could go to the bank and place a bounty on that players head. Usually if that player was someone who did this all the time many other players had already placed some of their gold as bounty on this player. I spent the better part of my days in UO hunting players who had high bounties on them and collecting the reward when I turned in their head. Also most players who killed lots of other players were known as Murderers and their name would be red to any other player and it would read, "
The Murderer John Doe" and because they were in that stage of player killing every time they were killed they would experience stat loss and gaining back those stats took a very long time when your name was The Murderer....