Note that if the number of captives is less than 80 men, it will not influence your traits. Thus, releasing them does you absolutely no good.

If the number is over 300 or so, it's likely not a good idea to release either as that is actually a hefty number that you'd not want to fight esp. if they retreat to a city that you want to siege.

Thus, there is a very small numerical window for release but executing is always awesome