He would have liked to say that it wasn't Isaac he was hunting. That it was only the Curse wearing Isaac's body. But then the other would show glints of his old self, and Hector knew it'd be a lie.
The problem was that the Curse hadn't changed Isaac altogether; it had taken his personality and twisted it, deformed him in most horrible ways.
Isaac was the one hurt by Hector leaving him; but it was the Curse seeking bloody revenge for it.
Isaac was the one who despised Rosaly out of jealousy; but it was the Curse choosing her as the perfect victim to bait Hector.
The Curse was to blame. But it was Isaac who would suffer for it.