@Joonhas
He is an outsider. He doesn't know the story of the world, he only know what is happening at the moment. So maybe he is a bit naive/hypocrite because he is not really active in the peace making but that's all.
He, like us, don't know what happened before.
Maybe (just maybe, it propably wasn't like that) just after taking the throne, the current demon lord sent peace letter and tried to stop the war but human kings didn't believed him (or choose to continue the war) and attacked the demons restarting the war. In that case, who is the guiltier person, the current demon lord who tried but failed or the humans who restarted the war and ended being beaten.
Other possibility, more likely and much older, at first, the demon were peaceful but the humans decided to attack/kill/destroy/slaugther them (we know how much humans can be racist against other humans so what about someone who isn't human at all ?), seeing their family being slaugthered the demons decided to counter attack, maybe it took several generation to stop, then the humans, grieving their lost parents and having forgotten (or chosing to forgot) that they were the one to start it, decide to attack once again. And here is the vicious circle of grief and slaughter.
What i mean is not that Baribossa (and other humans too) has to forgive the demons just like that but that slaughtering the demons here is not an act of righteousness but just to make oneself feel good without a care for anything (or it could also be the act of a psycho).
After all this, i still agree with you that the MC is "weird" because even when you are outsider if you are normal you would be angry/against/disturbed... about the demons' attack (at least in the beginning, before your reason come back to calm you) but here, the only time I saw him angry was when his wife was attacked, not an hint of sadness for the lives lost (not that i remember at least).