Okay, so, at first I thought at the end of the game that the whole murder plot was in Jack's imagination... but then there came the coda with everyone (except Jack) eating the salt and dying. Right after which it repeated the intro (which by then I'd forgotten) establishing that in fact there were bodies found at the bar... so apparently this part wasn't Jack's imagination. So it seems the salt was poisoned... which means the murder plot was real, but in attempting to save the husband's life Jack was inadvertently indirectly responsible for the deaths of Bob and a number of bar patrons instead. Though some of the fault here may have been Bob's, for convincing Jack that it wasn't real and that he wasn't really saving anyone; if not for that, Jack may have taken the threat of the poisoned salt more seriously and gotten rid of it before anyone ate it. (Then again, given that he's not exactly a pinnacle of responsibility, maybe not...) Hence the patheticness... not only can he not tell his delusions from reality, but when he actually does try to do the right thing it ends up going horribly wrong... as well as the tragedy.
As for his seeing the couple at the end as his own mother and father... I'm pretty sure that was just his hallucination again. The couple really were as he saw them at first. Even how he initially saw the husband didn't look like how the husband looked on the other end of the phone call, and was probably part of the hallucination... maybe he was at first seeing the husband as a younger version of his father, but didn't recognize him until he saw him as his father as he is now.
But, as I said, there still seemed to be something I'm missing... I thought I'd figured out how it all fit together, until I reread this thread, now that I'd finished the game and didn't have to worry about spoilers, and I saw this post (http://www.adventuregamestudio.co.uk/yabb/index.php?topic=40799.msg554448#msg554448), which raised a good question... which you replied to (http://www.adventuregamestudio.co.uk/yabb/index.php?topic=40799.msg554579#msg554579), but without fully clearing up the question it raised. So... I'm still not sure about the answer to that myself. What I now think may be the case... but I'm still not sure... is that it wasn't that the mother killed the father, but that the father killed the mother (which is, after all, what it looked like was going to happen in the first place). Sure, the figure behind the door said (s)he was Jack's mother, but obviously Jack's memories of the event aren't completely veridical, and certainly he'd be able to tell his parents' voices apart.
Still, even if that's true, it raises a couple of other questions. Why, in the vision, does the figure claim to be Jack's mother? And what is Jack's own culpability in this; why does the moose refer to his negligence? For the first question... ah, I had a tentative explanation, but I just thought of a much better one. In the case of the couple he's currently dealing with, the wife is plotting against the husband; therefore in his distorted recollection of his own family tragedy, Jack switches the genders to match them to this situation. (After all, he does in the hallucination in the end equate the couple to his own parents.) As for the second question... well, it could be that something Jack did enraged his father and precipitated his violence; the fault wouldn't really be Jack's, then, but as a child he could easily have seen it that way and exaggerated his own guilt. For what he did, the possibility most hinted at in his vision is that he perhaps (accidentally?) burned his father's "precious collection of fishing magazines"... he does remark afterward that that might be why his father is so angry; he doesn't say he's responsible for their burning, but it could fit with the moose's remark about his negligence. Though it might also have something to do with the birds? Actually, though, the fishing magazine thing could even fit in with why the fish appears as Jack's bad conscience... he associates the fish with his father, who would, given his crime, certainly be considered "bad"... yet Jack doesn't really want to acknowledge the crime and still feels for his father, which is why he's so reluctant to give the bad conscience up. (Why a moose for the good conscience, then? Well, maybe just because that was another trophy that happened to be handy (the fish started as a trophy on the wall, after all), so it made for a good match.)
So, anyway, I dunno; that's my take on what really happened and how all the pieces fit together, but I could easily be wrong.
Hm, one thing i'm still not sure of, though; one detail I haven't quite fit in yet: Jack asks Bob how he knew his father's number, and Bob says that's for Jack to figure out. Which seems to imply there's some significance to the fact that Bob knew Jack's father's number... but I'm not sure what the significance is. Unless... and here this is just a total wild guess, and by far the most speculative part of my interpretation (well, okay, it's all pretty darn speculative)... Bob knew Jack's father, and was in on the murder somehow, or at least helped Jack's father cover it up; this may even extend to Bob actively trying to keep Jack a useless drunk so he won't be in a position to try to bring his father to justice. In that case, Bob's death at the end might be a bit of poetic justice, though not so the deaths of the other bar patrons, I suppose...