Selection from –Political Treatise - Chapter VI, of monarchy (Page 1)

Spinoza's Words – (government by one man is not for the general welfare)

Because no one in solitude is strong enough to defend himself, and procure the necessaries of life, it follows that men naturally aspire to the civil state; nor can it happen that men should ever utterly dissolve it.

Accordingly, from the quarrels and seditions which are often stirred up in a commonwealth, it often happens that they change its form into some other.

Spinoza makes the following case against monarchy... 

If men lived according to reason all would do what makes for the general welfare. Those men recognize that no one man can be entrusted to care for all.  They know that leaders are never so free from avarice, envy, or ambition as to seek the general good over their own, especially as they are in the position of daily being subject to temptations.

But most men do not live according to reason and they think that it makes for peace and concord, to confer the whole authority upon one strong man. Kingdoms have lasted a long time whereas democraties are short-lived.  But slavery, not peace, is furthered by handing over to one man the whole authority. For peace, as he has said, consists not in mere absence of war, but in a union or agreement of minds.

Spinoza goes on... But in fact men are much mistaken, who suppose that one man can by himself hold the supreme right of a commonwealth. For the only limit of right, as we showed is power. But the power of one man is very inadequate to support so great a load. And hence it arises, that the man, whom the multitude has chosen king, looks out for himself generals, or counsellors, or friends, to whom he entrusts his own and the common welfare; so that the dominion, which is thought to be a perfect monarchy, is in actual working an aristocracy, not, indeed, an open but a hidden one, and therefore the worst of all.

Besides which, a king, who is a boy, or ill, or overcome by age, is but king on sufferance; and those in this case have the supreme authority, who administer the highest business of the dominion, or are near the king's person; not to mention, that a lascivious king often manages everything at the caprice of this or that mistress or minion.