We do get some sort of in-game cue with Yuria saying he used to be a sweet boy 9 years prior to the game. Then you have the beta notes (or rumored? There were two different lists on the site) having plotted out the whole Part III with Yurius "being saved by Yuria's love". Sometimes the original intention can show its true colors, though y'know, some things might've changed during the process but I'd say it still held some truth to it if the above was inserted in the game in the final version.
This manga showed him to be more humane and not entirely 'straight up ebilz for funz!1' possessed villain that is a young teenager. I mean when you think about how a couple of the holy weapons could overtake the caster's personalities (and twist them in the Lopt tome's case), then it's certainly possible the Yurius we see isn't wholly the same as to how he was originally before Manfloy decided it was time to be a dick and rip the family apart he so carefully 'made' himself |D.