Who was the first person to die of radiation poisoning?

The first fully documented case of true death by radiation poisoning was Marie Curie in 1934 after many years of exposure to different highly radioactive elements and x-rays from medical equipment she used in WW1 to examine wounded soldiers. Even her papers and cookbook are too radioactive to be safely handled without special protective clothing.

In 1932, a famous American socialite called Eben Byersdied after ingesting large amounts of a radium containing patent medicine called Radithor over the course of several years. However the real cause of his death was multiple cancers, not radiation poisoning.