He knows what's going to happen. It's as if he's seen the movie so many times he can't not act it out. His decision is a matter of fate. He's been excited and now it's too late to change things. Looking isn't enough anymore, and now he has to do something.
When we destroy ourselves we probably know exactly what we're doing, and yet at the same time, for the destruction to proceed without resistance, we have to be unaware. Tony Perkins knows his mother is an apparition, and yet at the same time he believes she's absolutely real.