Question about movie Quarantine? (If you have seen it)?

You know at the end where they go into the attic to find a way out and that guy has a bunch of rats.. That creepy creature eating the people in the attic, was that the old man who they hadn't seen for months and he was like some alien creature thing?? Or was that one of his creations?? Cause I think it was the old man who they hadn't seen for months but my parents think it was one of his creations or just some monster thing... Do you know what it was?