Deserters trade from city to city with 20 men. It would be a bad idea to attack our caravan.
You don't get anyhing when you kill a peasant, no items no gold. Why don't you let him trade for sometime and rob his profit? When you are a low level, you are no hero, and if you have no friends or clan you should have to stick with others in order not to get killed. (Killing a peasant or low level)
In PW, there's no purpose, but here you pay upkeep therefore you need to get money so you need to escort caravans etc. (Enforced RP)
If death penalty wasn't there, then it would be a mess. The fear of death is a good RP element. (Death penalty)