And there's the fact that Harry wasn't just killing Rudolph.
He was killing Rudolph slowly and cruelly, while savoring his terror. And that is the Winter Mantle's influence.
hehe, but, it's heavily implied the mantle and it's attendant influences are in fact your freudian Shadow speaking louder? I mean, check out what it did to Harry vs others known to have held it? Many serial killers with many different and distinct M.Os and preferences. I think that's Harry, Harry without ever touching Winter understood going after Michael's family to get to him. I'd compare it to a profiler, he's always understood bad things, he just doesn't chose to do them.
Though, I am making a specific distinction between mantle influence and influence that's changed how Harry will chose to behave. He has become a bit rougher around the edges.