I think the fact that they are taking their time proves they are looking at it as a murder. If they had real evidence against the snake, it would not hurt them, especially with public opinion, to move fast. When a human is suspected they are likely to take their time and not make their investigation public.
As for the legality of him have the snake and other animals, if you read the article I linked to earlier it says he was given those animals 10-13 years ago when he owned a "zoo". In 2006 he no longer ran the zoo and didn't apply for permits to keep the snakes and crocodilians after that. I think he would have been denied them without being a "zoo" and keeping them in a private residence from what I've read.