Vitiate is a verb that has several meanings: to make imperfect or impure; spoil or corrupt. To weaken morally; debase or pervert. To make a contract (or other legal document) invalid or ineffective.
In Herman Melville's Moby-Dick, Ishmael describes Queequeg, the heavily tattooed Polynesian harpooner:
His father was a High Chief, a King; his uncle a High Priest; . . . he boasted aunts who were the wives of unconquerable warriors. There was excellent blood in his veins — royal stuff; though sadly vitiated, I fear, by the cannibal propensity he nourished in his untutored youth.