villains in this manga:
"hey this country is shit, lets fucking destroy it"
*after everything is done*
"what we do now?"
"dunno, how about we ruining other country just for the sake of plot?"
"yeah let's do it"
"k.."
I do think Demonization makes people worse. They are mentioned by one of the grunts in the first demons invation that they have overflowing rage or something in them.
More or less, yeah. Strom specifically monologues about that when Shin confronted him after the knights sprung that trap. He said after he turned he stopped caring about humans. I'd say it's probably to varying degrees and might depend partly on what thing they were focused on when becoming demons. I'd say that female demon Strom may have personally knocked up clung to her devotion to him while turning, so you can see she's sort of in love with him or something. Zest and co. were clinging to some loyalty towards each other as well as their desire for revenge. All those commoners turned demon we've seen were basically in the worst states of mind when they got turned, so they're all just straight up assholes.