Per History.com, some of the first NASA (and NACA, the agency’s immediate predecessor) personnel to lose their lives were research pilots that had been testing jet- and rocket-powered aircraft. The purpose of the tests was to determine how the human body and vehicle would be affected at such speeds, with pilots such as Chuck Yeager setting unprecedented records. However the experimental nature of the latter led to a relatively high attrition rate among pilots (via phys.org). 

Those who did not perish in these early years progressed to become the first Apollo astronauts, but even then tragedy often struck. Four were killed in training accidents between 1964 and 1967, while the more prolific Apollo 1 disaster threatened the schedule of the whole project (via Smithsonian). After a fire broke out in the oxygen-rich capsule (the burned interior pictured above) and all three crew-members perished, for a time there was a very real possibility that the human and monetary cost would outweigh NASA’s goal altogether and even lead to cancellation. It didn’t, but only after an extensive redesign effort for the capsules so as to avoid a repeat of the tragedy (via Space.com).