Thus all virtuous deeds, inasmuch as they are referred to God, are professions of the faith whereby we come to know that God requires these works of us, and rewards us for them: and in this way they can be the cause of martyrdom. For this reason the Church celebrates the martyrdom of Blessed John the Baptist, who suffered death, not for refusing to deny the faith, but for reproving adultery. (St. Thomas, Summa theologiae, II-II q. 124 a. 5)
St. Maximilian Kolbe may have volunteered to die, in order that another should live, but the reason why he was in a consentration camp in the first place was precisely because of his Catholic faith. He "lived Jesus" by "laying down his life for his friends," therefore displaying the "greater love" Our Lord spoke of in John 15.