As Lord Advocate he was the minister responsible for the persecuting policy of Charles II in Scotland against the Presbyterian Covenanters. After the Battle of Bothwell Bridge in 1679 Mackenzie imprisoned 1,200 Covenanters in a field next to Greyfriars Kirkyard. Some were executed, and hundreds died of maltreatment. His treatment of Covenanters gained him the nickname "Bluidy Mackenzie". It has been argued that both he and Claverhouse kept to the letter of the law. It is unclear whether or not the epithet "Bluidy" is contemporary; it appears in "The Heart of Midlothian" (1818), given to Davie Deans. The language of blood prevails in the published testimony of Marion Harvey, hanged in 1681, who calls her blood onto Mackenzie: ""that excommunicate tyrant, George Mackenzie, the advocate", among others.