My take on the message of the movie ...

I think the movie conveys the message that people should act according to their passions and truly be the persons they want to be in life.

In the film, Lou wishes he were a fearsome, big-shot gangster; he even lies to himself to satisfy that wish. See the way Lou feels elated when Dave dishonestly says that someone in Vegas mentioned Lou. See the way Lou tells Dave that he killed many people in the past (when the truth is that Lou never killed anyone until the movie's end). See the way Lou talks about sharing a jail cell with a big-time gangster when, in truth, the men spent about 10 mins in a cell together ("That's it?," Sally asks). See the way Lou feels dejected when he couldn't protect Sally the way a big-shot gangster would have done.

Grace also reveals certain details about Lou's earlier life, which counters Lou's desire to be a big-shot gangster. Grace says something along the lines of, "Lou fetched coffee for the big guys in the old days," which means Lou was a low-ranking dude. She also mentions Lou's nickname as a coward, and she says that Lou ran away when Cookie (Grace's husband) got killed.

So basically --- Lou wishes he were a big-time gangster, but he never was a big-time gangster. There is therefore a disconnect between who he wants to be and who he actually is. A lot of pain comes from this disconnect. Throughout the film, his character imparts a layer of sadness. Lou soaks his life in dreams and nostalgia. See the way he talks about how Atlantic City was so much better in the old days ("Floy-Floy"; "You should have seen the Atlantic Ocean then"). The suggestion is that, unhappy with who he is, he lives in a past constructed by his imagination. He does so in order to escape from the present misery. In other words - he lives in the past to overcome the sorrow of being an aging, unknown man who lives inside a dingy apartment in a declining city.

The end really changes his life though. He musters the courage to kill two tough criminals, and he saves the girl. As an old man, he thus finally (FINALLY) becomes the big-shot, fearsome gangster that he always wanted to be. As a result, he experiences a tremendous amount of happiness with himself, and he finally wins Grace's heart. At the end, he is walking arm-in-arm with Grace, hinting that he will be happy and will no longer have to rely on nostalgia to get by in life (Notice how he and Grace are walking happily together at the same time that a building is being demolished? They don't care about the city anymore). The symbolism is that, by finally becoming what he wanted to be (i.e. - a gangster), Lou improves his life and his self-worth to the point where he doesn't need nostalgia and memories anymore.

That's why, again, I think the movie's message is that people should diligently be who they want to be.