Here's a bit of a left field idea. Just as drinking from a murky pond has a chance of making you sick...killing has a chance of making you mentally sick. Some may crumble at their first kill. Others may kill over and over before the weight of the actions weighs on their mind. So like getting physically sick, you get mentally sick. Lose your will to live. Mope (slow) speed movement. Needing more food to keep going. The shakes. Hearing noises/voices (like the current 'bug' of global direct messaging, you get random conversations of others played as 'voices'). Going to pull the trigger and instead lowering your weapon. Like I said, a bit of a left-field idea, but I think its an interesting idea that other's can probably take and develop a lot further than me.