How Can This Be?

Some of the saddest lines I have ever read by a Christian, let alone one of Luther’s status, are these:[1]

[In speaking of “holy martyrs”…] When they were called to arms even by infidel emperors and lords, they went to war. In all good conscience they slashed and killed, in this respect there was no difference between Christians and heathen. Yet they did not sin against this text. For they were not doing this as Christians, for their own persons, but as obedient members and subjects, under obligation to a secular person and authority. But in areas where you are free and without any obligation to such a secular authority, you have a different rule, since you are a different person.