@Merilirem,
@Guspaz: Na, Merilirem. I see your point, but he clearly over powered that warrior chick and wanted to slaughter her. Sure, they have been in his dungeon, but they could give him good reason. Somehow they ended up their by a teleportation trap (some other demons (his allies?) set them up) and they just want to leave that place. As this is a labyrinth, he could just send them off, without the security of his home base being compromised too badly. Wasn't this dungeon also known be the humans as his home? Instead he lived out his grudge against humans. He had a tragic past, that's for sure, but he became a racist. And what an arsehole of a racist. He wanted to kill them, because he didn't like their species. Too be fair, this is cold blooded.
I'm not sure, if they could have run, but the dragon could also let them go instead. And even though we don't know if the girl started it, she was beaten up so badly, he could have stopped at some point. Instead he was all "I won't forgive you!" And while I'm not a big fan of skipping the "Talk it out"-option, there is not sense in trying to talk it out with someone, who is trying his best at and being close to successfully killing you. (The girl was also trying to murder the dragon, but was not even close. He could have tried talking it out.)
Yeah, poor dragon, but in this situation, as you said "The MC was not wrong in fighting" and he got what he asked for.