I agree with the second paragraph, but dont forget the only reason he didnt kill her is because he was stoppped by his skeleton (who is a lot older than him so he has soe experience and wisdom to share because of that) and the "reward" i assume you mean the help with the defences, if that is the case it was explained earlier that this is for literally innocent people just like they were innocent and dragged there to be killed basically, should they end up as the same kind of people and forsake the innocent? thats just my take though.
I understand that he did it for the people, but it is exactly what the princess wanted.
The entire reason she summoned children as disposable soldier slaves, was for that .
He could still protect the innocent people with great ease AND punish the princess, she's not needed alive and/or healthy for the protection of the people to be achieved.
Changing mind because his skeleton summon, told him to "it's bad, mkay?", is in my opinion VERY weak plot-wise and just made to follow one of the most annoying tropes in Japanese manga and literature in general, of having the MC be a goodie two shoes regardless of how vile someone is.
The princess knowingly and intentionally sent him to die in the forest, and he experienced being eaten alive , all because the princess deemed him a talentless waste.(meanwhile the adults she kept imprisoned but safe, that tells you A LOT about her moral fiber and why she needs to be severely punished), call me crazy but I would have a HUGE grudge after such a mentally scarring ordeal, one that the words of anyone would not be able to quell.
I mean let's look at the facts here.
He didn't even bother to punish her at all and a couple of slaps or the silly "hur hur I'll make her think she's my summon" deal, is no real punishment in the least.
What did the princess do?
She summoned 207 children with the intent of keeping them as disposable slave soldiers, that were meant to be perpetually mistreated, to "keep them in line" as well as using the lie that she'll return them to their world as a reward. Meaning that her plan was to eventually "dispose" of them all, when she's done using them since they are too dangerous to keep alive for at least two reasons (one being that they are proof she broke the law and used forbidden summoning magic, that is punishable by death, the other is the whole story with the heroes turning into "demonlords").
There are 201 children's families who will potentially never see their children again, and those of the teachers as well.
She sent the MC to die in a terrible and excruciating way, because he was "powerless", then essentially ordered Funayama's slow and painful execution via starvation and abuse.
She murdered almost 50 people during her abduction (most of them children).
plus the abduction was the trigger that caused the MC's mother to off herself and his father (however bad their relationship was).
After all that the words of a skeleton, no matter how wise, should not be able to save the princess from a severe punishment.
Heck the MC is a very powerful healer, he can make her suffer greatly(say by throwing her to be eaten alive by goblins a few hundred dozen times) and still have her be healthy in body at least.