The lawgiver must assume for his purposes that all men are bad: ib.i.

3.
Villari has useful remarks on these principles in his Machiavelli, Book ii.cap.

iii.] Forms of government and religions are the personal creations of a single brain; and the only chance for a satisfactory constitution or for a religion to maintain itself for any length of time is constantly to repress any tendencies to depart from the original conceptions of its creator.
It is evident that these two assumptions are logically connected.

The lawgiver builds on the immutability of human nature; what is good for one generation must be good for another.

For Machiavelli, as for Plato, change meant corruption.