My Theory - SPOILER ALERT

Layla is a spirit guardian or angel sent to ease Billy's passage to the afterlife. In the end, he really does shoot himself and returns to Layla for his afterlife reward.

The football player/strip club owner was an evil man and Billy was the tool that God (Mickey Rourke) used to remove him.

This is the only logical reason that a woman as attractive and desirable as Layla would fall in love with a human serpent like Billy.