Walt shouts in agony looking at the damage his wife had done
The camera zooms in on the small cut on Walt’s hand. In many parts of the show, Walt had been shown as invincible and powerful, in charge of whatever situation he is in. Here he bleeds—showing weakness. It is reminiscent of scenes from other works in which a God-like character bleeds, revealing their humanity (for example, in the movie 300, when Leonidas pierces the “God-King” Xerxes' hand with an arrow)
This wound cuts particularly deep for Walt because it is delivered by his own wife. His original intention in manufacturing meth was for his family and he wanted to be the respected “provider” figure that he could not be as a schoolteacher. In being attacked by his own family, he is brought down from that pedestal and the tie between him and that family is irreconcilably severed.
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