>> but kosoul didn't give up.w there was another model home across the street, and he saw sarah walker arrive for work. >> oh, you know what? somebody just walked in. i won't be able to stay on the phone. >> prosecutors believe he went inside and tried to rob sarah. >> give me that watch. >> there was a fight. while trying to subdue her, kosoul bit her, then he used his knife and stabbed her more than 20 times and cut himself in the process. nelson villavicenzio and his wife were just outside while the murder occurred. had they stopped in to say hello to sarah, they, too, might have been victims. in october of 2006, kosoul was convicted of capital murder and sentenced to death. >> people will kill for the most trivial of reasons. reasons that no rational person would ever imagine. >> what the science did in this case is it made that defense attorney get up in his opening statement and say, my client was guilty and this was a robbery gone bad. he would not have said that if we hadn't had the physical evidence that