Why is everyone so confused?
They tortured the wrong man. When he breaks free and while Elise is using the bathroom you can very clearly hear him writing something! He DID suffer a head injury and had been tortured out of his mind to where he actually thought he did in fact do the things they said and wrote a suicide note and hanged himself.
Because he wasn't a sociopath but a remorseful normal person he felt deep guilt for having found out he did those terrible things that's why he wasn't violent and just wanted to end it all.
The other theories don't make any sense, those who don't get this must not have heard him scribbling on paper while Elise was in the bathroom.