So do you think Edward Norton's character (spoilers)...

...always intended to betray Nick or was it really because he felt disrespected?

I think he was always going to betray Nick. If the mild insults and "disrespect" seen in the film actually caused him to have a change of heart and take the scepter for himself then that just strikes me as poorly written character motivation. A few "No, we'll do it my way." from De Niro, a real professional and someone whose book he should take a page out of, and all the sudden change of plans. Not buying it. Maybe if they established his character more or wrote in more tension among the two AND Max I could buy it but not the way it's presented.