Why kill an 89 year old man in jail? Unless he was gonna rat it doesn't seem like a good move.
Three potential reasons:
Reason 1 - He obviously pissed a lot of people off when he was on the streets. He may have killed someone and the victim may have had a family member locked up who wanted revenge.
Reason 2 - Regardless of whether you think he was an actual snitch, or simply used Connolly for protection and never gave him real info, he was still considered a snitch by many people.
Reason 3 - Some young punk may simply have wanted some street cred for taking out a street legend. Simple as that.