He knows what all unidentified serial killers know: If you don't talk about what you did, you'll probably get away with it so long as you don't male any stupid mistakes.
Personally, I don't think he's a serial killer. I think he finally "let it out" or he was found out.... and it's just a matter of time before we all know.
And I think its very possible a second person may have been involved.....either directly or indirectly. I think we will eventually find out that it's a tragedy all around.