It is obviously not justified simply because it is possible somebody has more powers we don't know about. But that possibility does increase the risks of letting them loose or subduing instead of killing them. Are there powers of teleportation that might let them escape once they recover? It's a possibility that increases the risk to innocents. Since they already made pacts for domination and longevity, is it foolish to consider the possibility they might have escaping powers, or demon summoning powers if given the opportunity to sacrifice more innocents?
However, the unknown is outweighed by the known here (domination, killing, history of evil actions resulting in sacrificing innocents after exploiting them).
Their repeated history of doing these things indicate that if they escape they will probably do so again.
Again the implication that taking your time and generating a plan is somehow a bad idea. Now, bear in mind I am not necessarily criticising the decision the paladin made to kill the evildoers. I am criticising this particular argument on why they charged in and did it. It may be that cutting them down really was the only way to deal with them but it doesn't sound to me like anything else was really tried beyond "kill them all and let the gods sort them out" and THAT is not paladin-ly. The paladin code is not a document that requires only adherence to the letter without regard to the circumstances. If that was the case, paladins would be LN not LG.
20+ kids with dominate abilities statistically will clean that party's clock if they charge in there and try to hack them down without a good plan. Barring very good saves they are going to loose. My objection isn't so much with killing the Children of the Corn but with the lack of plan that led up to it. The party seems to have plowed headfirst into a fight with the odds stacked against it then realized the only way out was to kill the children to stop them and save themselves (and finally have the DM come to their rescue). That is not noble, that was simply a poor plan and not just on the paladin's part.
Tzarevitch
My understanding is the situation was "The evil kids who use mind control and sacrifice people to demons are escaping, what do you do?"
Having the paladin step up immediately and use lethal force seems appropriate in that situation.
Letting them go so that a better plan could be created, different planned spells used, and better information gathering was an option, but it would be risking them escaping and gaining more dominated innocent victims to sacrifice later and no guarantee of better information or an opportunity to stop them.