The Hitman is a gangster character in the merged holodeck programme of Emergence, who shoots the train's Engineer and carries a brick to Keystone City to set it in a wall.
The Hitman appears in the seventh-season episode Emergence. He is one of the passengers on the holodeck's Orient Express, the scenario through which the Enterprise's newly formed intelligence expresses itself.
When the Engineer tries to take over the train, the Hitman shoots him dead and takes from him a brick, which he guards closely. Deanna Troi and Worf follow him when the train stops at Keystone City, where he walks to a particular wall and fits the brick into a gap in it, declaring that he is laying the foundation.
The act coincides with the ship modifying a cargo bay to build something new, and helps Troi interpret the programme: the characters are working together to assemble an emergent life-form.