What are three instances in Native Son where Bigger switches from feeling fear to implementing some kind of cold, calculated action?
There are quite a few instances where Bigger Thomas goes from feeling fear to implementing cold, calculated actions. The first would be with the death of Mary. He accidentally kills her. This was done out of fear; however, the way he covers up her death was cold and calculated. He shoved her body into her chest to carry her downstairs and then realized a better way to get rid of the body was to burn her. When he tried to get her into the furnace, he realized she wouldn't fit so he cut off her head. She may have already been dead but he had no issue desecrating her body.
Another instance of his fear leading to cold and calculated action was when he killed his girlfriend, Bessie. He was afraid of getting caught and he knew that she could be the weak one who would cause them trouble -- either by turning them in or just by not doing everything he needed. She fell asleep and he bashed her head in with a brick. After that, he threw her down the air vent.
Another instance that is not as severe as the other two would be when he killed the rat in the very beginning. The entire family was jumping around, afraid, including Bigger. He took aim, and killed the rat. You could possibly even argue that this was foreshadowing the fact that fear could turn him into a cold and calculated man.