@Qelix
He became a speciest because the humans whom he once protected and loved brought an army to subdue him and turn him into a weapon. Then he joined the Demon King and his country of followers, who I assume ALL have similar stories - they were all slaves of humans, mistreated by humans, probably many survivors who lost loved ones to humans. The Demon King wanted to protect them and live separately from humans - it sounds like he didn't even try to fight the humans. Instead of letting the demons rage and go against humanity, he protected humanity from the demons' justly-deserved rage and anger, and protected the demons from the humans' greed and ambition.
And he was killed for it, and I imagine the country ruined. Because 'how dare our toys escape'. Black Dragon God lost absolutely everything at the hands of humans, and was trapped in that dungeon for ages, unable to avenge his friends or protect the followers of his best friend and lord. You're looking at this from the perspective of someone who is fully lucid and full of mercy and reasoning. Instead, yet another weapon-wielding asskicking human shows up in front of him, even in this lonely place where he is trapped. Why would he go, "Oh, you're probably a good person that I can just send on their way." after witnessing such evils that you and I can't comprehend? Why would he be what we consider reasonable?
For that matter you're approaching this from the perspective of a human. This is a dragon. Who is far, far older than a human can be. Your brain can't even begin to approach the understanding of living that long and having that many experiences. We also can't assume to know how a dragon is supposed to think. Saying 'well he could have done this' means nothing when we don't have that information. It can also easily be countered. 'Well, the humans could have NOT done this' and then none of this would have ever happened. Just because these people aren't the ones at fault doesn't mean much to a victim. He sees another of his tormentors appear, another butcher. To him, humans are the demons. Humans are the monsters.
Imagine how a human in a more stereotypical fantasy setting would react if an orc or ogre walked into their house with a giant axe. It wouldn't be, "Oh, welcome, are you here for tea? Or perhaps lost?" It'd be screaming and someone'd be getting out a sword and leaping to attack. Because that there's a monster, which your knowledge tells you is an existence that wants to hurt you. What does BDG's knowledge tell him? "The humans are here after me again. The humans who butchered my friends. The humans who murdered the people I was supposed to protect. The humans who betrayed me and chased me." Why would anyone EVER expect him to react casually and kindly in this situation? THAT is unrealistic.
MC did what he had to do. He had no other choice. But the Black Dragon God was not necessarily wrong in his reaction. Just over-victimized. I view neither of them as bad people, just a bad situation as a result of the evils of human greed.