Why did Saints Cosmas and Damian die?

A:

So little is known about Saints Cosmas and Damian that we can not really know how or why they died, or even whether they were real people. Most of our traditions about the twins date from the fourth century or later. The Catholic Encyclopedia says they were ordered to recant their faith and, having refused, were subjected to a whole range or tortures that failed to hurt them, and were eventually beheaded in 287 CE. The Catholic Encyclopedia concedes that much of the traditions about Cosmas and Damian are "fables".

The Basilica of Saints Cosmas and Damian is located near the site of the pagan temple of Castor and Pollux, whom Paul L. Williams identifies as the forerunners of the Christian twins.