How does Atrazine kill weeds?

It is by disrupting photosynthesis that atrazine kills weeds.

Specifically, the herbicide in question binds to a plant-specific protein. It thereby interferes with a plant's electron transport chains. These chains play critical roles in producing energy from sunlight and oxidizing sugars. Their malfunction will cause a plant's starvation and prevent a plant's carrying out life-sustaining activities.