I think that he was a teenager when he burned a cat alive. Usually teenagers make stupid things. I know people that make things like that and worse in their youth and they aren't murderers.
He is not a teenager when he bought a pair of handcuffs and iron legs. "This was to try something new with my wife" he said, laughting. His wife is jailed at the time. What a good surprise for her, when she is released. Not to mention the project of a torture room he described to his inmates.
It's strange that they don't considered the possibility of Bobby Dassey and Scott Tadych being the murderers or the "German" man: http://www.convolutedbrian.com/an-alternative.html