"The doctors realized in retrospect that even though most of these dead had also suffered from burns and blast effects, they had absorbed enough radiation to kill them. The rays simply destroyed body cells - caused their nuclei to degenerate and broke their walls." -John Hersey
Everyone (well, almost everyone) recognizes that radiation is bad for you. And the higher the energy of the radiation, the worse it is for you. The reason is relatively straightforward.
(Image credit: Environmental Protection Agency.)
When high energy particles (or photons) come into contact with normal matter, they knock the electrons off of atoms, ionizing them. This action breaks apart bonds, disrupting the structure and function of cells on a molecular level. And, as you might expect, the higher the energy, the more extensive is the damage that the ionizing radiation can do.
(Image credit: Uzm. Dr. Kamuran Kuş of Bilkent University.)
Targeted radiation -- at cancer cells, for instance -- is useful for this exact reason: it destroys the cancer cells. Sure, some of your cells are in the way, too, but radiation therapy is designed to kill the cancer faster (and more effectively) than it kills you.
But too much ionizing radiation ...