What is ironic about rev. parris' owning a slave

Reverend Samuel Parris was a Puritan involved in the Salem witch persecutions and trials. Arthur Miller wrote the play The Crucible, in which the trial hunts are brought to mind as compared with the McCarthysm, i. e., another form of accusation without evidence. The Reverend had a slave brought from Barbados; in the first part of Miller's play the Reverend finds his daughter was involved in pagan rituals in the forest, precisely the kind of thing the Reverend was intent on persecuting.