Threatt said 48-year-old Williams could be a nice guy, but Williams was also known as an angry white man who thought society gave preferences to black people.
Threatt is a union shop steward and had worked with Williams for 15 years at the airplane parts plant.
Threatt said he pleaded with Williams not to kill people at the factory Tuesday, but Williams pushed past him and fatally shot three co-workers at point-blank range. Two others were already dead in another building.
Nine people were injured in the shooting rampage.