Did he really steel the suitcase?

It's not abnormal to have two sets of car keys. It seems fitting that this character would, as he's a smart and meticulous fellow. I didn't view it as ambiguous or even think about that possibility until this thread. But I suppose it is a possibility, just like there are a few other scenes in the movie concerning the barmaid that could have been in his mind. My view was more cut and clear, though: He was going to take the money from the thieves and run away with the barmaid, but when she was a no show, that was the straw that broke the insomniac camel's back. He really had no reason to keep it then or to keep going on in general, because at that point he realized that life without love isn't much of a life at all.

It did not even occur to me that Titta could have fantasized about killing the men. This theory is consistent with one detail that bothered me: how could he have covered the car if he was hiding inside it? We see the hitmen uncovering the car right before Titta shoots them.

I think there's much more weight to the fact that it's imagined. Imagining this whole scenario would be very much in keeping with his character, acting like James Bond is very much against his character. It seems much more like the fantasies we all have where we wish we'd done something differently, been more brave, or pro-active in a given situation. Everyone wishes they were as cool as James Bond with hindsight. Also he says that he has a gun, but he's never fired it - he's not a violent man, and certainly not a murderer (contrast him to the mafioso who shoots the disabled kid in the back).

When he tells nitto that he has the case, the audience is as surprised as Nitto himself because we're thinking 'what the hell is he talking about? they nicked it, and his car keys and ran!', and then we see the events at the same time as he is sat before the guys. It very much gave me the impression that we were seeing his 'thoughts' rather than a flashback.

And think about what he says about bluffing when the bank are counting his money and he dupes them over the missing 100 grand. He carries through with absolute and utter dedication and belief. He is a master at bluffing, and this would be the biggest one of his life.

This leaves the question why? Perhaps the fact that he has 'never been loved in his life' but develops a relationship with the waitress which ultimately ends badly for him (he believes he has been stood up on his birthday), combined with the hatred of his existence, his spiralling addiction to heroin, and strained relationship with his children leads him to not want to live anymore. He knows what will happen, and perhaps he just goes willingly to his death, fantasising that the money he stole he has been able to give to the ex-hotelier and his wife who have such huge gambling debts.

I reckon that certainly in terms of lonliness, isolation, and shyness (the shy man notices everything, but is never himself noticed) this type of depression, imagination, fantasy cycle is extremely common and so that's how I perceived the whole bag thing. Truly awesome film!