Go to the corridor with cells and take an empty bottle near the Carl's bed. Go forward and talk with Murray about the soap. Use the bottle with Murray.
You come to the solitary confinement cell called 'The Hole'. There is no soap. Take a sponge and use it on the filthy wall. Read the inscription and click on the door. Talk with Murray about the inscription, he gives you a baseball ball. Hit the right wall with the ball and do this until you see part of the pipe (three times).
Use the crowbar with the nut pipe. The rat will comes and die immediately.
Throw the rat into the bucket. 48 hours later Randal will get a soap (pick it up from the floor). Wet the sponge with the dirty water from the filthy bucket and use the sponge with the door slot. Murray will come. Randal will leave the cell.