"There is another theory which states that this has already happened."

Sure, but place her in a military coup situation for example, and the brutality witnessed there would be small fry by comparison (IMO). I think the (whole) British public genuinely only got a small taste of what she would have been capable of if granted free reign.
I believe she could have rivaled any political monster of the 20thC, if the power structures in place at the time, had enabled it.