Maybe Bolvar is starting to forget who he is and is starting to think that the memories of Arthas and Ner'zhul are his own. So attacking Light's Hope was an Arthas move but he's starting to think it was his own thing. That's not to say that Bolvar isn't still there it's just it's getting hard for him to tell the memories apart. Especially when you visualize memories you don't exactly see it in third person. You see everything from a first person perspective.