Why should an enemy not try to finish someone off, making sure he stays down ? I would argue a seasoned fighter would not take the risk of someone getting up again, because this might lead to his demise. According to Historical medieval martial arts, as far as I know, this was done.

 

Standing enemies are much more important on battle field than downed enemies and often it is better just heavily injury enemies than kill them, because wailing enemies in ground lower morally of enemy troop usually more effectively than dead enemies and after battle wounded enemy soldiers force enemy army to spent their resource (food, beds, dressings, etc.) and men power to take care of them, where dead soldier get only buried or carried home. And wounded enemy soldiers if left behind by their 'friends' can be taken as prisoners and interrogate and then ransomed back or exchanged to prisoner taken from your side. Of course modern military practice don't necessary work in all the scenarios that come up in PoE (as there is dragons, giant beetles etc. things that modern military rarely fight against), but they at least give some incline why someone would not just finish off downed enemy.

Leaving Soldiers behind wounded is indeed more of a modern military practise. But appart from the fantasy setting, i don´t assume that the player party will even fight battles on a war scale. Fights are taking place on a much smaller scale where duel situations are more likely to occur. Of course, if preasured, it´s not a good idea focusing on the guy you just send to the ground but defending against the immenent threat in front of you. But if you have the time, it may be a good idea to make sure. And as said, historical documents suggest that this was in fact done. It´s not necessarily the act of a frantic lunatic or an feral animal. But you are right, there are situations where it would be more reasonalbe to take care of another target.

 

On a side note, I don´t how hard this would be to implement in the AI behaviour.

It would really depend on the specifics of the moment. Sometimes an enemy's best interest would be to go after the downed guy to make sure his downing's permanent. Sometimes it would be to ignore him and face the still-conscious threat.