Niek,

Twain did have a genuine love of cats.  I believe you have misread
Fanning's book.  Fanning theorizes that Orion's delegating the
duty of killing unwanted kittens to young Sam was a source of a
deep-seated grievance that Sam held for his brother.

For related items see Twain's cat quotes at:
http://www.twainquotes.com/Cats.html

and the testimonials from others who knew him at:
http://www.twainquotes.com/Bambino.html

Another related page that may be of interest:
http://www.twainquotes.com/Vivisection.html

Twain wrote numerous items condemning animal cruelty.

Barb