I like when Valtiel is taking over Walter and that's because that's when Walter becomes really in character. However, if there's one critique I do have it's this:
Walter isn't a little kid, nor does he have the mind set of one. He was chosen as the holy conjurer and according to some was the father of God. This is a man who did go to college at age 18; something I don't think could happen if he was acting so child like.
My request therefore is to have Walter grow up. He doesn't need Valtiel to be evil, because he already is. We're talking about a guy who tried to kill Eileen because she did something nice for him.
Notably I realize that Silent Hill is open up to interpretation.