Zack

He didn't. It was believed he was the killer's apprentice, and he admitted he was. He aided and abetted the killer by attempting to obscure his identity. He made a deal to be committed rather than go to prison. They thought he had killed one of the victims.

He later admitted to Sweets that he had never killed anyone. He isn't to Sweets that his admission was a doctor/patient confidence and he couldn't reveal it.

Finally, they were able to prove that he had never killed and his sentence was reduced. He was going to have to spend, IIRC, another 18 months before release.

Now, I think Zack was ill-used. Supposedly the producers had problems, again IIRC, had issue with the actor, Eric Millegan, though he did appear from time to time. If they did want to get rid of him I would either have killed him, or had him find a position in another institution. Either would have been more respectful of the character of Zach that had been established. The whole killer thing appeared out of whole cloth, which is not good writing.