If he were a hard man up for murder then the papers wouldn't make reference to how weak a man he was.
If he was a petty offender up for theft or burglary, the papers would have mentioned this straightaway: "How could a man up for a nonviolent crime like burglary or drugs possession be locked up with a violent psychopath."
So the only answer is that he assaulted either a woman or a child sexually.
If that is the case, he was gagging for the treatment he got I don't feel the slightest remorse for him.
An important question and one that I should have asked myself first. I did some frantic Google work following your query. I hit over 12 web-sites. Again and again there was no mention of any thing as to his crime.
Finally I got IRISH EMMIGRANT ON LINE. It then stated, he was "serving three years for assault". Still no definition though. I fear you are likely right and appalling though it is, a terrible justice came to him.
Still hurts to think of the sordid mess in that prison though.
Makes the US military system of prisons, for terrorists look like a sunday school treat, I must say.