Kid knew that was Shanks fleet. Kid was facing the enemy. Kid chose to charge up that attack. He could have sent his metal arm flying, coated it in haki, jumped out of the way, literally anything... but he just couldn't react.
He failed to react to a Shanks that came straight from his front in a literal straight line... that came jumping from behind a fleet several kilometers away... and Shanks cut through his railgun, through his charged power, through Killer... and wasn't affected by the explosion despite being right there as well.
It was fair. Kid simply failed in the IQ department, in the speed department and in the durability department and got owned like he deserves.