Why did the scientist *spoiler*

Well in the change room they did tell him that they would release him that night into the water I think. Also, he probably thought that the creature was released already and wanted to take a jab at that confidence that Strickland always had about him. That is why he was laughing when he told him that it was the cleaners. Strickland was always looking down at others for being weak or unimportant so this was a chance to teach him a lesson.