I don't think the policeman not killing the terrorist had a large effect on the number of casualities.
Most people who died were outside.
One terrorist went inside whilst shooting a kalashnikov. When the policeman shot him, he detonated the bomb.
Because of the shooting, people started running outside where two more suicide bombers were waiting and they detonated themselves outside the entrance/exit.
Quit a "clever" plan if you think about it
@Colwyn, in most Turkish airports you can't enter the arrival part of the building. There is security at the door and they won't allow you in. You need to go throught the departure entrance and therefore through the security checks and then make your way to arrival. If you are allowed to enter the arrival part of the building directly, there will be security as well.