Aside from that, I don't really mind that the MC is rather ruthless towards the humans. He has a point and it makes sense, he has no reason to show mercy. Those very humans wanted to kill him as a baby simply for his black hair and eyes, the humans in this world are obviously both twisted and moronic(Although the whole premise that black hair and eyes is somehow linked with being evil or cursed is rather dumb as well if you ask me). If he were to show mercy and let them go, it'd probably bite him in the ass later, seeing as it was the King who ordered for the purge of the so called "Damned". So they would report that there is a strong one in the forest and they'd bring back a big army instead. And suddenly a lot of the MCs family would die from a war, caused by him showing mercy to people he should not have shown mercy.
However this approach of his does have a flaw as it doesn't change the problem at hand; that Humans think of spirits as nothing more than objects and slaves. The goal of the MC ought to be peace, as this cycle will never end, and the humans will just expand more and more over time until they'll start to want to expand into the place where the Spirits live.
The ones in the comments showing dislike, saying "Why doesn't he just use ancient magic to erase/change their memories instead of killing them? He's living with ancient spirits! Surely they know would know such magic!"
First of all, why would that be a matter of certainty that they know such magic? Second of all, memory-manipulation magic opens up for a whole bunch of weird possible plot-points. If you know someone can remove or change your memories, you might start to question everything you know.
It also doesn't really solve the problem; if they simply remove their memories and dump them outside the forest, people would obviously figure out that there's something weird going on and they would investigate one way or another. It would probably be preferable if they think of it as a particularly dangerous forest rather than a forest where you go in, and then end up confused and memory erased outside the forest a few days later.