The reporter avers that he was a “rapacious, hated mining magnate”, but what exactly did he do wrong?
From the link.
“Twain didn't mention that one of Clark's Montana opponents, Henry Huttleston Rogers, had rescued Twain from bankruptcy. Rogers and Standard Oil cronies set up the Amalgamated Copper Company, which defrauded shareholders. As an insider, Twain profited from the Amalgamated deal. Twain cast his essay as if he was offended by having to listen to Clark drone on at a banquet, but his wallet may have been talking. Twain and co-author Charles Dudley Warner coined the term “the Gilded Age” in their 1873 book by that name.”
There was a big scandal [payola] about his ascent to office as senator; Bettering the condition of others wasn't his concern. Clark cut timber on federal land, and he benefited from Arizona's "deportations" of union men who were kidnapped and driven out of state. Criticized for the sulfurous smoke and denuded landscape from his mines, he said, "Those who succeed us can well take care of themselves."
"Robber barons," some historians call the tycoons of that era. Others prefer "industrial statesmen." Unlike Carnegie or Rockefeller, Clark left little charity, only corruption and extravagance.
"Life was good to William A. Clark," wrote historian Michael Malone, "but due to his own excesses, history has been unkind."
Credit: William Merritt Chase, 1915, Corcoran Gallery of Art, Washington