Mary is often called the "Queen of Martyrs" because of her own martyrdom at the foot of the cross.
It might seem odd to think of Mary as the “Queen of Martyrs,” when she did not suffer a painful death.
However, according to St. Bernard of Clairvaux, Mary suffered her own martyrdom, not of body, but of spirit.
He explains his thoughts in one of his sermons featured in the Church’s Office of Readings.
The martyrdom of the Virgin is set forth both in the prophecy of Simeon and in the actual story of our Lord’s Passion. The holy old man said of the infant Jesus: He has been established as a sign which will be contradicted. He went on to say to Mary: And your own heart will be pierced by a sword. Truly, O blessed Mother, a sword has pierced your heart. For only by passing through your heart could the sword enter the flesh of your Son. Indeed, after your Jesus – who belongs to everyone, but is especially yours – gave up his life, the cruel spear, which was not withheld from his lifeless body, tore open his side. Clearly it did not touch his soul and could not harm him, but it did pierce your heart … Do not be surprised, brothers, that Mary is said to be a martyr in spirit.
St. Bernard continues his sermon, explaining that Mary can be considered a martyr even though she was not killed for the faith.
Who are you and what is the source of your wisdom that you are more surprised at the compassion of Mary than at the Passion of Mary’s Son? For if he could die in body, could she not die with him in spirit? He died in body through a love greater than anyone had known. She died in spirit through a love unlike any other since his.
St. Bernard believed that she could be considered a “martyr in spirit” on account of her intense love of Jesus and the pain she suffered at the foot of the cross.
With this in mind, it is fitting that Mary would be called “Queen of Martyrs.”