It could be added in the future where you can choose between the two, but for development stages it should be only thieves. Is there any idea of how you could make it playable with an option of being a thief or a guard? It is a really good idea, but thinking of a lot more ideas makes my brain not able to focus very well.
I was thinking that at the beginning of the game (Say in a lobby for example) You are able to choose between sec. guard and thief. Maybe only 8 can be guard and 6 can be thieves, I do not know. The guards can have knock back sticks for example or a wooden/stone axe. They could upgrade by gaining points. These can be earned by killing thieves or protecting the loot for a certain amount of time.
The guards can possibly roam the map looking for thieves. If it is a building in a city, the guards can be limited to the building only, while the thieves get full access to every area of the map. This means that the guards Must stay around the treasure.