Shoot at mouse cursor

Hey guys,

I'm having a little problem with my current unity project. It is quite difficult for me to type out the problem, so instead I have made a short video showing my problem.

Video

This is the current script

HeadRotate.js

 #pragma strict
 
 var position : Rect; var crosshairTexture : Texture2D;
 
 function Update ()
 {
     var position = Input.mousePosition;
     var newposition = Vector3(position.x,position.y,-camera.main.transform.position.z);
     var lastposition = camera.main.ScreenToWorldPoint(newposition);
     
     transform.LookAt(lastposition);
 }
 
 function OnGUI ()
 {
     position = Rect( Input.mousePosition.x - (crosshairTexture.width / 2), (Screen.height - Input.mousePosition.y) - (crosshairTexture.height / 2), crosshairTexture.width , crosshairTexture.height );
     GUI.DrawTexture( position , crosshairTexture );
 }

If anyone can help to fix this, please answer.

Thanks,

JPiiONEFOURONE