One of the most famous anecdotes about Spinoza is that after the (1672) massacre of De Witt brothers, he was inclined to place a sheet of paper at the murder site with, "Ultimi barbarorum" (ultimate barbarian) [see, e.g., Della Rocca]. Leaving aside the veracity of the story and what it purports the reveal (here's Deleuze), it also helps frame how readers interpret Spinoza's (1670) Theological Political Treatise.* This is, in addition to its other agendas, commonly taken to be a kind pro-De Witt (or anti-clerical and anti-Orangist) Republican intervention (or, at least, the final five chapters).+ Fair enough.
Now consider this passage from the (1677) posthumously published (and incomplete) Political Treatise, where Spinoza is describing the malfunctioning of an aristocracy:
In every council the secretaries and other officials of this kind, as they have not the right of voting, should be chosen from the commons. But as these, by their long practice of business, are the most conversant with the affairs to be transacted, it often arises that more deference than right is shown to their advice, and that the state of the whole dominion depends chiefly on their guidance: which thing has been fatal to the Dutch. 8.44.
Maybe I am missing something, but this strikes me as a pretty clear analysis of De Witt's trajectory and downfall. (Admittedly, De Witt's family background was more patrician than Spinoza would allow if he is thinking of De Witt, but De Witt's rise to power was a consequence of much richer patricians scheming on his behalf.) Spinoza here resists the urge into turning De Witt into some noble kind of a Cartesian philosopher-king. De Witt was arguably one of the most talented mathematicians of his generation and guided Dutch politics for a generation; nor does Spinoza turn De Witt into a martyr here or into a morality tale about the dangers of the mob. Rather, Spinoza explicitly treats De Witt's power, and subsequent fall, as evidence for the bad institutional design of the Dutch (oligarchic) aristocracy of his age (unless I am all wrong that this passage is describing De Witt).**
There is also a more subtle, speculative issue at stake here; that is, the fate of De Witt reveals that Cartesian philosophy has not generated, and may be incapable of generating, a genuine art of political rule. I am not sure that I can prove this is Spinoza's position (although I believe it his), but if it is not his, then I adopt it as my own. For De Witt was the star Cartesian student of his generation.++ And part of his prominence was his ability to talk as an intellectual equal with, if not superiority to, the social elite of the Dutch Republic (which included other star mathematicians and natural philosophers such as Huygens and Hudde). His fate and the near collapse of the Dutch polity is also a sign that the (Cartesian) new philosophy needs reform if it is capable of providing blue print for political art (recall here, here, here).
Anyway, it obviously does not follow that Spinoza was not outraged at the murder of De Witt brothers; nor does it follow that the Theological Political Treatise is not a republican, anti-clerical tract. But I do think that the passage reveals that Spinoza may have been less admiring of De Witt and even the Dutch Republic as such.
*Here's Feuer's interesting angle connecting it to Temple.
+ If you read Dutch you should take a look at this lovely blog post.
** Throughout the Political Treatise, Spinoza treats war or internal strife (and lawlessness/criminality, etc.) as de facto evidence for bad institutional design/functioning.
++ De Witt was a student of Frans van Schooten, who published De Witt's Elements of Linear Curves work in his 1661 edition of Descartes' Geometry (see here for nice details).