(SPOILERS) Is there ambiguity in the ending or did I miss something?

[MGC]

Jack killing Lana (the prosecutor) isn't actually depicted: he shoots the gun, but we don't see what he hits. This can completely change the meaning of the final (post-credit) scene. I'm not saying it's definitely true, I'm saying it can't be discounted, so we are left to wonder.

Yes, Jack killed everyone who was a direct threat to him. Captain Carter (of the police chief competition) threatened to turn him in if he doesn't leave (and Jack had nowhere to go, so it was effectively either Jack or Carter). Lilly (the sheriff) & Henderson (the soup smuggler) had to go too, as they conspired to kill Jack.

Notice that these deaths (that you could argue were about self-defense and survival) are all on-screen.

Now we are lead to believe that Jack snapped from one betrayal too many and turned irredeemably evil. He killed everyone who even knew his identity, no matter how innocent. Lana (mentioned), Arthur (the smartass drug dealer held captive by the smuggler), Fry (the 20 000$ guy), Moreno (the chocolate / painting chatterbox), Emma (the secretary).

But those deaths are all implied to have happened off-screen, through some ominous dialogue, like the reference to the slaughterhouse in Fry's scene or Jack's oddly menacing tone of voice in Emma's scene.

But how do we know that Jack snapped? We know this because... he killed Lana. Which we didn't really see happen.

(This is even more neat, as Lana's death scene is partially depicted, with Jack on-screen and Lana off-screen. There is matching ambiguity whether Jack is really shooting her for the sake of survival or because she is only an imagined, paranoid threat. But this is likely reading too much into the aesthetic.)

So is there anything I missed that completely ruins this theory? It seems at least not-impossible to me and I think it was very deliberate on the writers' part.

[Padelboot]

I agree that is is not impossible to interpret it this way. However I think it's quite clear that Jack is getting rid of everyone who could identify him as Mr. Boyd. Yes, Lana's death is not directly shown, but she was shouting at him the entire time, music swelling dramatically and after the shot: quiet. Everything is quiet, including Lana. No panic, no more anger. To me that signals that Lana was indeed killed.
In addition this is not the only "clue" to show that Jack is off his rockers. Don't forget the menacing soundtrack in the next day or that he is hallucinating the appearance of old officers from Freeburg, that cannot be there. Or the fact that the minigame you are doomed to lose tells you to save Jack's soul, aka saving Jack from completely losing all morals he had left. This is the day Jack hit rock bottom and to me the game is at least not ambiguous in saying that he is.
Also major spoiler for the spin-off Rebel Cops: Jack appears as the Sheriff again and in the very end kills the cops and Zuev, with whom he made deals before. It's quite clear that he doesn't have any qualms of getting rid of anyone in his way anymore.

Also in the phone call between Jack and the Colonel after Moreno was done for... something tells me that the Colonel knows something is up. I can't prove this one, frankly, but Colonel Henderson is shown to be smart enough and experienced enough to figure out that there is something going on. The behaviour of the guys bringing Arthur Sherman to Boyd and Arthurs frightened appearance also implicate they know Arthur isn't going to make it through the night.

Another thing is noticeable: Jack comes in to work late in the last days. Particularly on some days he arrives at work around 17:00. Why has he been so late? The first time he slept in due to Lana arriving at his door step. That one is clear. But the other times? It's not directly shown, but it gives him enough time to arrange for the bomb off-screen and getting rid of other evidence (like the bodies).

Long story short: I think it's made clear that Jack did indeed kill them all, even when it's not explicitly shown. There are other clues showing that he did do so. I would also add that in his eyes all of those were self-defense. After all... the FBI was hunting for him and not even his friend Ethan was willing to help him. They all knew who he was, so... yeah. He made sure he was safe(r) from being found out.

[MGC]

Yeah, you're right, many more things indicate that Jack was very troubled. I also tend toward the standard interpretation of events, as there is more support for it, but at the same time I think that the mystery is there and it's there for some reason, if only to leave options open for the continuation of the story. If the writers want to "resurrect" any of these characters, they can. I've yet to play Rebel Cops, so I don't know what happens in it.