Fatalah said:Walt focused on his reflection and began to pound his fist on the paper towel dispenser. Was Walt upset that he turned himself into a monster, for no reason?Walt hitting the paper towel dispenser.
Was the remission news bittersweet to him?
Or did the downtrodden man pound the dispenser in a celebratory fashion? One very important win for a man that has had few victories in life.
We don't need to spoiler tag anything from episodes that have already aired, right? In any case, comment #387 already mentioned remission...
I think all three of your explanations for Walt hitting the paper towel dispenser (remission news bittersweet, upset at turning into a monster, celebratory) don't make much sense.
I think it was more "Jesus, my stupid behavior almost got us killed for no good reason."
The good news at the end was ironic because Walt sees himself as the man of science and reason. He mercilessly mocked Jesse throughout the episode for not thinking, and also displayed a patronizing attitude (as he usually does) towards Marie when she was talking about analyzing CT scans, of all things. Yet it was his irrational knee-jerk reaction to seeing a reflection of the CT scan (that he lacks the expertise to properly interpret!) that almost got them killed.
Regarding the monster part: I don't think Walt sees himself as turning into a monster. He just thinks he's doing what he has to do for his family, and believes the ends justify the means. He will probably have that realization eventually if he continues down this path, but it has not happened yet.