Hal,

I addressed the traditional idea of Twain's so-called "dark writings"
coming out of personal issues in a paper I presented at the MLA
conference in D.C. a few years ago.  It's online at:

Mark Twain on Imperial Washington, 1900-1910
http://www.boondocksnet.com/twain/mla_mtdc.html

Here's part of the concluding paragraph:

>> When Twain's views and his preoccupation with imperial Washington
are considered, he does fit Jaher's description of the cataclysmic
writers. He faced personal tragedies, but he was also "obsessed with the
idea of holocaust," "joined abortive crusades" and "belonged to defeated
movements," and envisioned a social struggle with "a defiant force
involving society in its own destruction." Although the creation of an
empire brought many of the issues involved to the fore in 1900, his views
on many of them can be traced back to much earlier works. This
suggests that his later works should not be seen solely as reflections of
his personal tragedies but as statements of widely shared beliefs about
the political and social transformations of the age. <<

Jim Zwick