In a Meta example, most fantasy books usually deal with Heroes trying to fight Villains, in order to save the world. This books shows what happens if the Heroes went too far, and now it's up to the Villains to save the day from the Heroes, who unknowingly set the world on the path of destruction.
The 'Heroes' in the story are more psychotic and evil than most of the villains but still have all the powers of Good and are so successful at being Good that they risk destroying the world in a singularity of Light and Goodness. How is that possible? Mizzamir, and by extension those who follow him, is a Good person but not a good person. He has, consciously or not, figured out that the world he lives in has an objective Character Alignment system and that by performing Good actions - sparing the lives of enemies, redeeming villains, building thriving communities, and battling the forces of evil - he retains the powers of Good despite the context and reality of those actions - Cruel Mercy via Fate Worse than Death, Brainwashing for the Greater Good via Mind Rape, a Police State powered by Sinister Surveillance, and straight-up genocide and xenocide using Always Chaotic Evil as an excuse - being downright monstrous. Mizzamir and his supporters are not heroes, they're Rules Lawyers engaging in wanton Loophole Abuse, which is why Sir Pryce, a genuine hero, turned on them.
It takes three characters to keep the villain heroes on the side of good: Sir Pryse the knight, who was good and stayed good but was outlawed; Kaylana the druid, who is neutral and understands the trouble the world is in... and Robin, who witnesses for himself how the villains keep together and despite being a spy sent by Mizzamir, recognizes his right to choose his alliance and allies himself with the villains.
The villains who were whitewashed - there are two possible horrible scenarios:
1 - Before everyone became all nice, what's to prevent a lynch mob from forming if a whitewashed criminal, who honestly had no memory of their past crimes, encountered their past victims? Most of the time, a criminal could go "But for Me, It Was Tuesday", and would fend off the mob. But if they had no memory of any crimes, and were unable to even protect themselves, they might end up dead! However, it seems to be implied that Good people have become nicer, and thus probably forgiving. The authorities would also not likely allow this, since it was their policy.
2 - After the whitewashing is removed, are there going to be a lot of violent deaths? Granted, maybe not, but there is one female assassin, Cata, who had been whitewashed, and when the other assassins located her, she was married to a farmer, with two kids - could she be both a dutiful wife/caring mother and a cold-blooded assassin? Or would she kill her husband and the kids? Also, would she go after her previous targets, whomever they were?
Crossing over with Body Horror is the description of an emaciated owl, who is unable to kill to eat, yet is unable to die. The Fridge part is that there might be other predator animals, like foxes and falcons mentioned after the Dark Gate was opened, that might have been in a similar situation, where they too, despite not being Good or Evil, were unable to hunt for food or die themselves.
Fenwick's pursuit of Kaylana, and his plans for her, to say nothing about trying to get her to have some aphrodisiac-spiked wine sounds a lot like the actions of a stalker who won't take No for an answer.
Mizzamir's own actions show that even a Hero can do some very Evil things. What's worse is the fact that he doesn't even see it as having done Evil Deeds for a Good Reason.
Given that it seems that even one who is a Non-Malicious Monster could end up on the wrong end of an adventurer's sword, it would be pretty scary for a young werewolf, who has done no wrong, to go walking through the woods, taking a basket full of pork and lambchops to his grandmother, only to encounter a scary swordswoman in red. Of course, the book basically states as much (any werewolf would probably have been killed on sight).
The starving but unable to die owl, along with presumably every other predator, is probably why the druids are true neutral and desire to maintain the balance. They're the only ones aware of the impact an imbalance has on the world, particularly since they can talk to animals.
If the world would end once the last evil is eradicated, then it appears that the last evil could not possibly be eradicated, because the world ending would be the last evil. Of course, In-Universe, nobody may realize that.
The forces of Good might not even believe (if they knew it was going to) that the world ending in this way was evil, given their other views of things. It's discussed in the novel, where Kaylana says that even assuming they knew, they'd be compelled by their Good natures to fight the evil characters, even if they're saving the world.
If predators can't kill (mentioned on the YMMV page in Body Horror-book citation p. 215) how come the heroes can? Shouldn't it affect them more since they are "Good" (the above suggests killing is deemed inherently evil, even to live) and not simply neutral like animals (or druids)? They can also survive on plants, unlike most animal predators. It seems like the suffering this inflicts would be evil too...
Character Alignment system and that by performing Good actions - sparing the lives of enemies, redeeming villains, building thriving communities, and battling the forces of evil - he retains the powers of Good despite the context and reality of those actions - Cruel Mercy via Fate Worse than Death, Brainwashing for the Greater Good via Mind Rape, a Police State powered by Sinister Surveillance, and straight-up genocide and xenocide using Always Chaotic Evil as an excuse - being downright monstrous. Mizzamir and his supporters are not heroes, they're Rules Lawyers engaging in wanton Loophole Abuse, which is why Sir Pryce, a genuine hero, turned on them.