There are lots of intriguing titles that treat this development, 2 of which
I will mention here:

The Culture of Cynicism: American Morality in Decline. By Richard Stivers,
(cultural history and criticism)

And

Seeing through Cynicism: A Reconsideration of the Power of Suspicion.  By
Dick Keyes  (this last one is more of a Christian response to the problems
of cynicism in our culture)

Personally, I would attribute much of Twain's darkness in his final period
to the traumatic grief associated with deaths of his family members, coupled
with his own declining health.  That said, I see much evidence for his
maintaining hope and kindness and joyfulness in those years, some of which I
document in my own book on spiritual crises.

Harold K. Bush, Jr.
Saint Louis University