I say 60/40 suicide/murder.
His crime was obvious, even captured on film IIRC, and shaking-down drug dealers isn't the sort of crime that threatened the higher-ups, so there doesn't seem any real need to "shut him up forever". And, why wait 3-years when he could have easily been shivved in the showers by someone long before.
Besides, there exists CCTV video of him alone in his cell when this happened.
However, as he was accustomed to a life of luxury before his imprisonment, his depression may have built up to the point where he literally couldn't take it anymore and a knotted cord seemed a way out.