While making someone's kid help you dispose of their body is an absolutely abhorent act, it makes some sense, tactically. If you involve someone, especially a young person with limited experience with the law, they should be less likely to talk to the authorities. Additionally, dude didn't give her much choice but to make it a double death. (This is not the first time i've heard of this kind of thing. Never, ever date a chick in a true crime book.)
I'd agree this guy is non-fixable & should be removed from the gene pool by any means available.