The main theme in this film is the journey from innocence to adult hood. Everything that happens as the movie progresses is a symbol or representation of how David is "growing up" and dealing with reality and who he really is. In the beginning of the film David and his friend Jack are very lighthearted and have a carefree attitude about the world. They don't take many things seriously even when adult around them are trying to warn them, however cryptically, about the dangers of real life.
After the attack on David and Jack, David not only has to deal with the death of his friend, he has to deal with finding out that he is a coward who ran away and left his friend in his time of need. All of the times when Jack's spirit shows up and talks to him, telling him that he has to kill himself because of what he will become could also be interpreted as David's own guilt at what he did to his friend.
The thought of suicide would be a common reaction to feelings of guilt after the death of one so close.
As David becomes the werewolf, or monster, this could also be interpreted as his horrible feelings about himself coming out. He would think that he was a monster, inside, because of what happened to Jack. The pain he experiences when transforming into the werewolf could be attributed to the pain he feels over the loss of Jack. However, David does not deal with this pain, this reality of himself. He tries to ignore what happened and what is happening to himself. Every time except for the last that Jack appears to him, David tries to tell himself that he is crazy, writing off these warnings from his friend as his over active imagination after a traumatic event. He also tries to escape this reality through love with Ms. Price, using sex and pleasure as a means to avoiding his new reality.
Even something as mundane as the Disneyland references throughout the movie, the little Mickey Mouse dolls, is a reference to David and Jack's innocence and serves as a constant reminded to David that his innocence is gone. He now has to face the real world, and he will have to face it without his good friend Jack.
The ending could be taken to mean many different things. As Price is talking to the werewolf David, she tells him that she loves him. When she does this, it seems to the viewer that the werewolf stops snarling, and his face relaxes, perhaps some small piece of David's humanity coming through. However, in the next second he jumps at her as if to kill her but the police shoot him and he dies. I see this action as an act of sacrifice or even suicide. He was not jumping at Price to kill her, but he was trying to get the police to kill him. Whether he just wanted to die to not have to deal with this new reality, or he wanted to die to save other people like the woman he loved, we may never know.