Walterian thoughts
Maybe Walter kept/keeps himself drug-addled deliberately, because he, with good reason, doesn't trust himself when sober and in full possession of his capabilities. In a way, in his current situation he surrendered initiative to his son, whom he's using as a surrogate moral compass (perhaps not the best of ideas, considering).
A fully sober Walter might decide that he's not ready to let his universe die, and thus try to figure out a way to make sure the other universe does. Which is precisely what Walternate is doing, except that Walter had and still has a better working knowledge of multiverse technology.
Maybe Walter did arrive to that conclusion, but, heavy with the responsibility of it all being his fault and unable to "fix" things by killing an entire Earth, he stopped short of acting on it, and instead chose to shut himself down (besides cutting down small parts of his brain for good measure). After all, he knows that a) his knowledge makes him unthinkably dangerous, and b) his judgement is at best questionable. Separating the former from the latter by breaking his own mind in carefully planned ways seems like something that he would both choose to do and know how to.
A fully sober Walter might decide that he's not ready to let his universe die, and thus try to figure out a way to make sure the other universe does. Which is precisely what Walternate is doing, except that Walter had and still has a better working knowledge of multiverse technology.
Maybe Walter did arrive to that conclusion, but, heavy with the responsibility of it all being his fault and unable to "fix" things by killing an entire Earth, he stopped short of acting on it, and instead chose to shut himself down (besides cutting down small parts of his brain for good measure). After all, he knows that a) his knowledge makes him unthinkably dangerous, and b) his judgement is at best questionable. Separating the former from the latter by breaking his own mind in carefully planned ways seems like something that he would both choose to do and know how to.