"Martyr" is from the Greek word "witness". The early Church was famous for witnesses who were true to the end (death by persecution). There were so many, that martyr became synonymous with death for the faith.
This is very different from those who die while killing, as in Islam. The ideal Christian martyr is peaceful and gentle, like a lamb led to slaughter.
Felix Manz is in line with our recent coverage of Baptism.
During the Reformation, there were people who pressed for more thorough reformation. These people are generally known as the "Radical Reformers". The best known of these are the anabaptists (a derogatory term from their enemies, meaning "re-baptizers").
Anabaptists have only a idealogical connection to modern Baptists. Their more direct descendants are the Mennonites and related groups (now know more for non-violence and strict separation from the world).
The anabaptists believed that baptism should follow belief, which is what the systematic reading of Scripture shows. They also believed in non-violence (particularly the avoidance of military service) - largely because, at that time, military service was connected to religious warfare.
"Felix Manz became ... the first Swiss Anabaptist to be martyred at the hands of other Protestants."The baptism (and religious warfare) issue is very tightly connected with Postmillenialism, which I will address in a separate post.