Well no, kayn isn’t a serial killer , he has a dark mind but he just has an insane amount of willpower , his one arm is almost fully taken over in his model and it’s only because that arm was injured and rhaast healed it (if I remember correctly, it’s just that the darkin , though immortal ascended beings were not super strong in there minds from the ascension , like nasus didn’t become more brilliant from ascending

The darkin were no different before they got sealed into weapons , and some were not inherently evil though most were a bit brutal and went very much insane (aatrox) after being forgotten for so long since they are immobile as weapons