Bill Johnson is a former US Marine soldier and an excellent sniper. He was unwilling to work with The Organization. When he is under Carla's orders, he has a mission that consists to execute a unamed target on a ferry boat.
The experiment sniper gun that Michael retrieved with Victor is give to Johnson, so he can accomplish his mission.
We never know what is the target of Bill Johnson because he is killed by a booby trap that is set up by Victor.