AB E -- the quote isn't found on twainquotes.com and best I can tell, did
not originate with Mark Twain.  It has taken on a life of its own with
attribution to Mark Twain.  However, searching google for the quote with
attribution to someone else besides Twain using these search terms:

"poor fight back" + violence -twain

yields over 84,000 results.  The true source of that quote might be buried
there somewhere.

It appears to be another case of "if it sounds good, Mark Twain must have
said it."

Barb