In Islam it seems that "martyrs" can be defined to include those who intentionally take life. Specifically, those who die killing, or during battle with, enemies of the faith. Does Catholicism or any Christian denomination define martyrdom or sainthood in a way that can include those who take another's life? Has anyone been declared a martyr or saint by any Christian denomination for taking another's life?
To clarify the definition of martyrdom, in both Islam and Christianity, martyrdom is achieved by dying, not specifically by taking a life, although in some cases one's death may involve taking the lives of others.
The Catholic Church has certainly canonised individuals who died fighting for the Christian cause, but sainthood is not open simply to all who do so. Martyrs are generally regarded as Christians who die through persecution, but a heroic death in battle could also qualify as martyrdom - for example, King Oswald of Northumbria is considered a martyr because he died at the hand of a pagan while defending a Christian nation.
Hershel Shanks says in Christianity and Rabbinic Judaism, pages 88-89, in 1095, when Pope Urban II preached the First Crusade, he undertook to bestow a plenary indulgence on all who took part, promising that a crusader who died in a state of penitence for his earthly sins would be received into heaven immediately. In a speech attributed to Urban, he promised "We now hold out to you wars which contain the glorious reward of martyrdom, which will retain that title of praise now and forever."