Who was the girl and why was she seen in Wrecked?

While he struggles with the hallucination girl before he "kills" her, he says "you didn't think I would really do it but I did". Earlier after he first hears about the robbery he has a "memory" of him pointing the gun at the woman as if she is a bank teller and he is robbing her. By the end of the film we know that he never really pointed the gun at her because he was just an innocent bystander who got held hostage and taken away. All this leads me to believe that because of the radio announcement of the robbery he believes he is one of the robbers. Because of the false memory of him pointing the gun on her he thinks he may have killed her in the robbery. So when he keeps seeing the hallucination girl and she leads him to the car again he gets angry and "shoots her again" because he doesn't want the guilt of seeing her over and over. In the end we find out she was probably his wife or fiancé (he's wearing a ring the whole movie). And he probably felt guilty and kept hallucinating her because they argued the last time he saw her. That also explains why she always looks angry in his hallucinations.

Lack of food and water is causing him to hallucinate. He and, probably his wife, had an argument before he was nabbed. After waking up in the ditch, everything that happens to him is a mixture of hallucination and reality. Clarity finally hits him when he is brought face to face with the robber at the end. He never shot his wife.

I was thinking that he killed his wife well before the accident, but he cannot remember and her spirit appears after the accident very angry and want to make he suffer too for revenge. That's why she tries to confuse him at different times, but she disappeared when he killed her spirit in his imagination to survive. And the dog that guides and protects him was the spirit of the dog he had when he was a child.

Remember the scene minutes before when he was kidnapped by the robbers, he saw her sitting and waiting for him and he saw her with a surprise, like he didn't expect to see her there, so this is when he saw her spirit the first time.

From my understanding, there are a few things at work here. The first thing to do is read the pretty good summary of the film.

To answer your questions: Why does he try to kill her? He realises he is hallucinating and that everytime he sees her and follows her he is led back to the car. He is frustrated, tired, confused and quick to anger, which is why he shot her (in his imagination at least).

Another theory is that its a manifestation of guilt/anger as he spends some time in the movie believing he is the criminal. Perhaps as he gets angrier he feels that since he's already committed these other crimes, he can commit this one too. It's possible, although I prefer the first suggestion in the paragraph above.