You know what's stupid? It's not that he doesn't seek revenge, it's that he helps the kingdom and people who stole everything from him and even tortured him. Okay, he doesn't want to take revenge, you know what is the biggest fuck you, you can do? Travel to enemy kingdom or empire and offer your services to them.
Now the crux. If he was just let go because of his "curse" it would be somewhat moderately okay for him to cooperate with the kingdom to some extent BUT his kingdom showed that it is as immoral and evil as supposed demon king let's recount:
1) Kingdom uses an orphans as a warriors
2) Kingdom discarded their hero as soon as he became a threat and/or achieved his objective
3) They conducted vile and painful experiments on him (remember he was 9-10 years old)
4) Kingdom conducted terrible experiments on convicts and some other poor souls
Now this fucking intellectually impaired author wants to tell us that "Hero" is good? I ask you my fellow is a person serving evil with good intentions considered evil? In my opinion anyone who serves under evil umbrella despite their inner convictions is evil.
Next point, he just let go a thief, he let go sorry excuse of a noble who tried to kill him, he let go Dr.Mengele(C'mon it's obvious where author took inspiration from). Is he good? No, he is a dumbass. If he was 8y old boy living in a bubble, surrounded by loving family that shields him from realities of the world I would understand his actions. I cannot consider it realistic when a 12y old boy who lived in the orphanage, killed, massacred and lived among soldiers has this mentality......
What is more mind boggling is that it is SEINEN i.e. 14-16 and up genre demographic. Are japanese children fluffy like that in those ages? I would understand if it were shounen and to pass censure they needed to create this cringy ass story but it's seinen.
Lastly it is edging fetish manga, where you see a lot of fanservice but 0 progression, where jokes repeat all the time. I'm not for pedo stuff but they already made it so, barely one step? No they have already crossed the line, i don't know about you people but i don't consider 12y boys bathing with women as an appropriate thing.
If author wanted to create a more believable story about everything here, he should have made MC 8years old OR 6 years old then every supposed sexual edging action could be replaced with maternal action and his "morals" could be at least justified since he was doing gruesome stuff before he even understood what he was doing and listened to every BS nobles told him.