They didn't tell us but it was obviously something very bad apparently involving children. I'd guess it was actually him raping them as if he wasn't in it himself they wouldn't be able to tie him into the crime but I guess he could argue they had been doctored or were inadmissable due to how Donna got them - no search warrant etc. So, he'd have got off and come after April - hence Donna taking the chance to kill him. Still, you'd his associates might still target Donna's family as revenge unless they are glad to be rid of him too!