This is an example of paradox. Donne and the metaphysical poets, sometimes referred to as the tradition of Wit, were fond of using paradox in order to surprise and confuse their readers. Here, the paradox is in imploring God not for mercy, as is common, but for violence. It's not that violence cannot be associated with God (he also deals punishment, after all); however, it is still surprising that the speaker woud request violence for himself instead of grace, compassion or mercy.
