I can understand his thinking so: He had known her, hung out with her, been intimate - how could he feel she would go this far?
Even the tire slashing and emails to Lisa don't indicate this: Most (but not all, of course) stalkers never progress to killing. He likely thought it was stupid, jealous harassment. Even when he told others he could wind up dead, he couldn't seriously have meant it, or he would have taken serious action.
I cannot imagine his horror when the whole process started. Too bad his screaming did not alert the neighbors! Just imagine a neighbor or room mate having been alerted by this, and calling 911 and maybe even saving him! (and having her arrested). Of course with the AC on and windows closed, this did not occur.