6 ideas
Because a hereditary prince cannot be chosen, the only lever a society has over the quality of its ruler is his upbringing. The tutor therefore matters more than the bloodline: good character must be planted early, before flattery and privilege harden the mind, since a ruined prince ruins the whole state.
The prince is reframed as a steward bound to the people's welfare, not a proprietor of the realm. The test that separates a king from a tyrant is simple: a king governs for the benefit of the governed, while a tyrant governs for his own benefit, whatever titles either one holds.
Power systematically distorts the information reaching a ruler, because everyone near him gains by pleasing rather than informing him. Flattery is poison dressed as friendship, and it can come through praise, through art and panegyric, or through ministers who echo the prince's desires. The prince must deliberately cultivate truth-tellers and distrust anyone whose advice always agrees with his appetites.
War's real costs fall on ordinary people, who lose their lives, their property and their morals, and these costs almost always exceed any gain a prince can claim. That holds even for wars with a just pretext. A Christian ruler should exhaust arbitration, negotiation and even concession before fighting, since a peace bought at some price is usually cheaper than the cheapest war.
Good rulers cut crime by removing its causes, such as idleness, poverty, bad education and luxury. They rely less on harsh penalties after the fact. Laws should be few, clear and aimed at the public good, and punishment should be the last resort, used the way a surgeon uses amputation only when a limb cannot be healed.
Subjects copy what the ruler visibly does far more than what he commands. His private vices therefore become public ones, spreading through the court and down into society. A prince's personal conduct is a form of legislation, and his moral failures are multiplied across his whole realm.