I have read the novel and theres really no explanation why Gio or the other Gios are like that, Its just his personality.
And about the eating part, theres also an explanation for that but I think the manhwa will tackle that far later
From my understanding of the novel, a likely reason is because of the assimilation of his personality and memories as Giovanni was still pretty much in the process of growing. You could say he's 'incomplete' instead of broken. Plus, he's really just the type to easily accept things since it already happened like when he found out he died as Seo Gio and became a portrait. It didn't take long for him then too.
!! SPOILERS !!
!! SPOILERS!!
For Giovanni's death, it was an active choice he made. Him and Iser were captured after people found out about their bond. At some point during captivity, Iser grew really hungry. Gio felt that since he was dying anyway, why not offer himself as food? And so, Iser ate his teacher. It's a truly gut wrenching moment and I hope the manhwa does this scene justice.
Anyway, as for Iser's sister. She doesn't just want to erase the Sun God cult but the entirety of humanity as well because of what happened to Gio and her brother. Both her and Iser have grown to hate humanity so much so that they submerged their dimension and soon Earth
Is Giovanni kind or is he just broken? He speaks about his own brutal death like it's someone elses business. If you get murdered, normally you get mad at the perpetrator.
Also, if he was eaten by Iser then why his sister wants to erase Sun God Cult? Did Iser deceived her?