First generation Protestants were more likely to be heretics than those born generations later. St. Augustine explains:
“But though the doctrine which men hold be false and perverse, if they do not maintain it with passionate obstinacy, especially when they have not devised it by the rashness of their own presumption, but have accepted it from
parents who had been misguided and had fallen into
error, and if they are with anxiety seeking the
truth, and are prepared to be set right when they have found it, such men are not to be counted
heretics.”
newadvent.org/fathers/1102043.htm
That being said, this in no way takes away from the holiness and heroic virtue of those who died seeking to extirpate heresy in various lands. For example, a great many of the English martyrs, who died seeking to bring England back to the faith, have been beatified and canonized in recent decades (most notably during the pontificate of John Paul II.)