If Walts I am the danger monologue is where we see him becoming detached from reality, his breakdown in the crawl space is when all ties to his old life are irrevocably snapped. Having been backed into a corner by Gus, Walt attempts to use his stockpiled money to escape Albuquerque only to find his wife has used it to bail out her old flame. The bleakness of the situation causes Walt to snap as he laughs manically in the crawl space. It is after this moment, after Walt has seemingly accepted his grim fate, that the gloves are off and no act is too evil or too out of character for Walt.