There could be a relations penalty with the faction/village/clan whose people you kill after they've surrendered.+1
I came here to suggest it !
REASON : instead of chasing down non combative units that did not really run away and get captured, it is a time-saver and cooler (RP) that they just surrender (drop weapon, kneel or arms up). This just saves time at the end of battles against units that no longer fight anyhow and do not really run away.
The player can decide to kill them anyways (RP choice as not profitable really).