Vito starts the assassination by meeting with Don Fannucci, negotiating with him to lower the cost of protection, and succeeding. Shortly afterwards, Vito kills the Don and takes the money back.
My question is, why did Vito bother showing up with less money than was asked of him if he was going to kill and rob Fannucci anyway? From the conversation prior, it sounded like Vito and company had enough money to meet the original demands. Vito would have still met with Fannucci to pay him so the assassination route or plans wouldn't have changed. By negotiating, Vito risks potentially failing, angering Fannucci, and possibly getting roughed up or disabled before he can retrieve his gun (if not killed outright, it's unclear how much muscle or authority Fannucci actually had when Vito killed him.)
Why did Vito bother negotiating?