It's sloppy, agreed. I believe this is what's referred to as a retcon. But I'm still thrilled that they're saying he didn't actually kill anyone.

I re-watched the scene between Sweets and Zack and transcribed it for someone else. If you'd like to read it, here it is:

Zack: I haven’t actually done it before, you know.

Sweets: Had sex?

Zack: Ended someone’s life. Why doesn’t anyone ever believe I’ve had sex?

Sweets: You confessed to plunging a knife into a man’s chest.

Zack: No. I said I killed him – which I did; I told The Master where to find the man.

Sweets: But you didn’t plunge a knife into the man’s chest?

Zack: It wasn’t me.

Sweets: Zack, why did you confess?

Zack: I would have done it. If The Master had asked, I would have done it.

Sweets: No, no. You don’t know that. People have no idea if they’re capable of ending a life until they’re put in that situation. In all of our sessions, I’ve had question marks because you, at heart, are not a killer.
(Sweets starts dialing his cell.) I gotta, I gotta tell Dr. Brennan this.

Zack: You can’t tell them. Because I’m your patient, and you’re not allowed, ethically.

Sweets: Zack, don’t you want your friends to know that you didn’t kill anyone?

Zack: I’m still an accessory to murder. If you tell them, they’ll take me out of here and put me in prison. Hodgins assures me I would not do well in prison.

Sweets: OK, what about the person that actually did commit the murder? He’s still out there.

Zack: No. The Master killed him. So he could recruit me. There could only ever be two.

Sweets: You have to let me tell the truth.

Zack: You can’t tell anyone without my permission…. We should go in; I don’t want to get you into trouble.