Mackenzie originates from a settlement west of Nuka-World. Sometime before Nuka-World's Overboss Colter took over the theme park, his gang attacked Mackenzie's settlement. She successfully negotiated a ceasefire in order to tend to the wounded on both sides, including Colter himself.
After Colter secured the theme park, he returned for Mackenzie and offered her a deal: work for him and his gang would leave her settlement alone. She accepted his offer, put on an exploding shock collar, and works as Nuka-World's doctor in the Nuka-Town market.
Even though she is a doctor and has shown a commitment to some form of the Hippocratic oath, having provided life saving treatment to the raiders who were attacking her settlement and to her captors at Nuka-World, Mackenzie is eager to be rid of the raiders and is not bothered by the prospect of the Sole Survivor killing them if it means freedom for the Nuka-World traders.
Her dialogue and tone remains pleasant and respectful if the player character chooses to remain overboss but is tinged with fear. If the player character chooses to complete Open Season, she becomes immensely grateful and will thank them profusely, expressing hope that the park's residents can look forward to a safe and prosperous future.
While she, along with most of the traders, was enslaved by the raider gangs, she freely chooses to remain there if the player character eliminates the raider gangs and works to make the park a better place for everyone.