They did not go to the Russians place with the intent to kill him. They were only there on Silvio's behalf to collect the money. Things went sour from then on as they were not prepared to kill the Russian right. The brain surgery makes sense, makes a good excuse to say "I don't know what happen"

"I have this Nightmare. I'm on 5th avenue watching the St. Patrick's Day parade and I have a coronary and nine thousand cops march happily over my body." Chief Sidney Green