oppressor n.

late 14c., oppressen, "to press unduly upon or against, overburden, weigh down," also figuratively, "overwhelm overpower" (of sickness, grief, etc.); also "burden with cruel, unjust, or unreasonable restraints, treat with injustice or undue severity, keep down by an unjust exercise of power," from Old French opresser "oppress, afflict; torment, smother" (13c.), from Medieval Latin oppressare, frequentative of Latin opprimere "press against, press together, press down;" figuratively "crush, put down, subdue, prosecute relentlessly" (in Late Latin "to rape"), from assimilated form of ob "against" (see ob-) + premere "to press, hold fast, cover, crowd, compress" (from PIE root *per- (4) "to strike"). In Middle English also "to rape." Related: Oppressed; oppressing.

It is the due [external] restraint and not the moderation of rulers that constitutes a state of liberty; as the power to oppress, though never exercised, does a state of slavery. [St. George Tucker, "View of the Constitution of the United States," 1803]