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So Walter's actions in the first season, apart from maybe the initial bad idea of making meth for a quick buck, are entirely rational and relatable. Everything that happens to him is the result of that initial bad decision and he handles it (barely) in the same way most of us would in that situation, which is why it is so compelling.

As the show carries on, much of his actions are, likewise, just dealing with or making the most of the situation he has found himself in. But it is also sprinkled in with a few things that you really can't say you would do in the same situation. It starts a bit morally ambiguous, like letting a woman (who is ruining the life of your surrogate son by getting him back into using drugs) die from her own bad choices. Then it gets a bit less morally ambiguous, like exposing a child to a poison (that he is saved from) to manipulate that said surrogate son.

But for the most part, his choices are kind of made for him by the circumstances and justified by trying to protect others from dangerous people. But then at a certain point he has successfully changed his circumstances, eliminated all threats, and he is 100% free to walk away scott free as a very wealthy man in good health with an intact family, as are his allies. But his transformation is truly completed at this moment. Instead of seeing this as his opportunity to escape that life, he sees his opportunity to take the place of the men who have exploited and endangered him for years. And he convinces his allies to be a part of it. And, as a result, he eventually ruins the lives of every single person around him because of it, including himself.

The point is that there is no understanding what he did in the end. Much of it was out of his control, but all of it ultimately happens because of his personal weaknesses, his ego and his greed. Even the initial decision to make meth was completely driven by ego because he couldn't accept money from an old business partner for his medical bills because of ego. By the end, that is all that is left, him, his ego, and his meth.