Let’s see if I get this right. Fear makes you angry and anger makes you evil, right?

If the memories of my youth serve me anger 'leads to the dark side of the force' via the intermediary 'hate'. That is, it leads you to go around frying things with lightening and choking people with a force grip. This is only 'evil' when you do the killing in cases where killing is not an entirely appropriate response. Unfortunately humans (and furry green muppet 'Lannik') are notoriously bad at judging when drastic violation of inhibitions is appropriate. Power---likely including the power to kill people with your brain---will almost always corrupt.

But then, in “Return of the Jedi,” Lucas takes this basic wisdom and perverts it

Not nearly as much as David Brin perverts the message that Lucas's message. I in fact do reject the instructions of Yoda but I reject what he actually says. I don't need to reject a straw caricature thereof.

“If you get angry — even at injustice and murder — it will automatically and immediately transform you into an unalloyedly evil person!

Automatically. Immediately. Where did this come from? Yoda is 900 years old, wizened and gives clear indications that he thinks of long term consequences rather than being caught up in the moment. We also know he's seen at least one such Jedi to Sith transition with his own eyes (after first predicting it). Anakin took years to grow from a whiny little brat into an awesome badass (I mean... "turn evil"). That is the kind of change that Yoda (and Lucas) clearly have in mind.

All of your opinions and political beliefs will suddenly and magically reverse.

That seems unlikely. It also wasn't claimed by the Furry Master. Instead what can be expected is that that opinions and political beliefs will change in predictable ways---most notably in the direction of endorsing the acquisition and use of power in ways that happen to benefit the self. Maybe the corrupted will change from a Blue to a Green but more likely they'll change into a NavyBlue and consider it Right to kill Greens with their brain, take all their stuff and ravage their womenfolk (or menfolk, or asexual alien humanoids, depending on generalized sexual orientation).

Every loyalty will be forsaken and your friends won’t be able to draw you back.

Except that Lucas in the very same movie has Darth Vader turn back to the Light and throw Palpatine down some shaft due to loyalty to his son. Perhaps Lucas isn't presenting the moral lesson that Brin believes he is presenting.