Aware of Sinatra's animosity, Brando purposely worked to get under the Crooner's skin, and
would intentionally mess up scenes because he knew it would get to Sinatra.
Since J.R. was such a monumental turd it could have been anybody, but it turned out it was Kristen Shepard,
his sister-in-law and mistress portrayed by Mary Crosby(daughter of legendary Crooner Bing Crosby,)
who stepped into the villain's office, pulled the trigger and left him for dead.