I think the guards did it.
I agree with you. These guards have low-paying jobs in lousy conditions. Whoever wanted him dead was desperate and has a LOT of money. Easily could see 7+ figures offered to take him out. I can't believe we don't know yet if there was a camera in the hallway to see who went in or out of his cell...impossible that there was not in this sort of facility.
Shut up and eat your gruel.