Prisoner Tommy reveals to Red and Andy that his cellmate at another prison had claimed responsibility for the murders for which Andy was convicted. Andy approaches Norton with this information, but Norton refuses to listen, and when Andy mentions the money laundering, Norton sends him back to solitary confinement.
If they kill Tommy for small reason but why they didn't kill Andy for big reason?
Because there was no need to kill Andy after they killed Tommy.
Tommy would had been a witness if his case was to be reopened. Now that Tommy was killed Andy could not open his case.
And Andy, being a banker, covered his financial frauds pretty well. Norton needed Andy to do his dirty work and he thought he would set him right after solitary. He had the control of the diary in which every account was recorded.
Norton didn't felt the need to kill Andy because he thought he was in complete control of Andy's situation.
Norton and the guards were making money from Andy's work. Tommy was killed so that Andy would stay in prison.
They didn't want to kill Andy, because he was making money for them.
