You know, I can see how things go, with the current situation stacked heavily against him because even his own allies are thinking that he might be monster with human skin (which he is, fyi the city he's in is heavily persecuting his kind because one time they tried to let one of his kind live in the city as a trial, due to the monster blood containing chaos, shits went off like little boy in Hiroshima and countless lives lost), so he got no allies around and everyone's out to kill him and his son if they found out the truth.
Hmm, I think I finally find the reason why my own MC will help out a kingdom he hates, also the kingdom that will eventually betray him by initiating a nuclear detonation to kill him.
So far in the very distant plotline the story will have my MC help out a bunch of refugees of a third country who got stuck between two warring nations, a bit out of goodwill but mostly for the selfish reason to make himself feel good after the lives he took. Rations dwindle and he ended up using "Sharing", a skill that he thought is to allow him to distribute his own lifeforce and mental strength to those in need, in order to continue to sustain them. He might have superhuman longevity, but spreading his lifeforce and mental strength over less than a ten thousand weak and feeble people still takes a toll of him. Shit hits the fan when he got banned from entering a city belonging to the same country as the refugees and the city lord orders them to be killed or leave, at this point the MC's hatred flares up and cue World War Z where the Zs climb up the wall moment.
It is at this time do my MC know that the skill isn't "Sharing", it is "Assimilate". As he eats up the city lord (yep, cannibalism yay), he knows the truth about the city's plight and they are unable to let the refugees in because they too are low on ration. Then with the shared perception of the refugees he has, he sees them biting off meat and chomping onto fellow men like dogs, babies whose wails end abruptly as their mothers' as they all share the fate of getting consumed alive by MC's accumulated hunger.
This I think is good enough as a guilt trip, city level massacre by cannibalism plus the souls (ie memories and personalities) of those who are refugees and rhose who get consumed are stuck inside him too.