To avoid suspicion from a child who could become a powerful wizard who grew up thinking that and never questioned it at all until a demon told him it wasn't natural causes. Even then, he doesn't seriously question it.
White court vampires in the book never bother with that, they trust it looks natural enough and the mortals find some natural cause.
By the time Malcolm died papa Raith was aware of Margaret’s curse. He would have killed the child if he knew about him. He did not. Maybe he did not even know about Malcolm. He only learned everything in blood rites.
Justin had a reason to leav Harry alive.