@cicerone imposter,
Hawkeye foolishly, and snobbishly, talks about the films Williams was "reduced to doing" in the past few years, as though he has some unique insight into the types of film roles Williams
wanted to take after he turned 60, when, of course, he is ignorant on that score. Mainly what aging actors want to do is to go on working, and creating diverse characters, and Williams seemingly encountered no problems doing that--not only in films, but on TV, as well as on the Broadway stage, over the past few years. He wasn't "reduced to doing" anything that justifies Hawkeye's demeaning appraisal.
In the 2013 critical dud, "The Big Wedding", Williams fellow cast mates included 3 other highly regarded Oscar winners--Diane Keaton, Robert Di Nero, and Susan Sarandon--all of whom must have been equally washed up and "reduced to doing" this movie according to Hawkeye's standards.
In fact, Williams had just been in several successful films as well. Notable among those was the critically well received "The Butler" (2013)--in which he played President Dwight Eisenhower. He had also just reprised his role as President Teddy Roosevelt for the yet to be released Night at the Museum 3, and there is no reason to believe that this film won't be as extremely commercially successful as it's predecessor in the series.
Williams appeared in 83 films and 27 television series-- he was nominated for the Academy Award for Best Actor three times, beside winning an Oscar for Best Supporting Actor. He also received two Emmy Awards, four Golden Globe Awards, two Screen Actors Guild Awards, and five Grammy Awards.
He had a long, and distinguished, and successful career, right up until his death, despite Hawkeye's rather absurd and melodramatic attempts to portray him as a pathetic failure at the end. He never stopped surprising us with his genius for comic improvisation, or with the range of dramatic roles he chose, which is why we never lost interest in him, and why he never lacked for new projects.
Williams seemingly had everything to live for--a loving family, good friends, devoted fans, financial and career success, and a solid record of accomplishment, both artistically and in terms of his philanthropy, of which he could be justifiably proud. He had so much to draw upon in terms of support for whatever he was dealing with or going through. Unfortunately, those things weren't enough to balance out whatever internal pain and darkness caused him to leave us.
Hawkeye, in his usual know-it-all arrogance, thinks he knows the answer to "Why?". More thinking people, respectfully recognizing William's complexity, as well as his acknowledged vulnerabilities, know that he took the only answer that matters with him.