Please point out when, between the time the Romans expelled their monarch and set up the Republic, and the time the Republic was dissolved to become the Empire that Rome had a monarch.
Does the word "Caesar" not translate to "king" in your textbooks? Didn't the fact that it became an Empire, imply an Emperor? Did you miss the whole "overruling the Senate" thing that Brutus's friend performed?
The downfall of the Republic is exactly why the Founding Fathers took care to NOT mimic the ideal old Roman Republic.
157-86 BC Gaius Marius: I seem to recall this man here enjoying a rather dictorial rule for some time.
I will say it was maybe a poor choice of wording, to say "monarch" with exclusion. But the rule of one man over the proceedings (dicatorship) of the courts and laws is what our Founding Fathers did not want to happen.