I view McCarthy’s alcohol abuse (which was probably the strongest determining factor in his death) as a reaction to the long term struggle that he carried on, nearly singlehandedly for the better part of five years.
In defense of his country, he sacrificed his reputation, and eventually, his life.
The left killed him as sure as if they had fired a bullet into his brain.
I spoke to a Democrat officeholder and historian and he told me alcoholism was an epidemic in DC amongst officeholders of both parties (less so today). Unless it was on such public display that resulted in homicide (Ted Kennedy) or just an embarrassing situation with your stripper girlfriend (Wilbur Mills), if every "alcoholic" had been outed, you'd have emptied out a huge % of Congress. I believe there was a gentleman's agreement between the parties to not bother passing proverbial judgment on the members' drinking habits. That's why it was so audacious when ex-Sen. John Tower's alcoholism was put up by the Dems front and center to defeat him for Defense Secretary. The hypocrisy was epic.
In McCarthy's case, it was probably both the stress of the job, but also that drinking was so much a part of the DC culture.