If you’re able to grasp how easily some people are taken hostage by their psychological defense mechanisms, it makes perfect sense that the only way you can reach them is, paradoxically, to validate them in what you can’t help but regard as their wrongheaded perspective. Yet at the same time you need to get across to them that you don’t—and can’t—agree with what they did.
Appropriately executed, what such ironically supportive corroboration does is not have you actually concur with their viewpoint but acknowledge that it feels genuine to them. Erroneous or not, it’s held with sincerity and, more than likely, with considerable conviction too. In a word, from their mindset, it’s authentic. So to the degree that addressing a person in this sympathetic way accurately reflects their reality, they’ll be left with very little to defend against. And as a consequence, taking such an accommodating approach will increase the possibility that they’ll eventually admit to something that otherwise they’d stubbornly refuse to.
Here, despite the aversive effect their actions have had on others, you’re ascribing to them benign (vs. aggressive or malicious) intentions. And frankly, it’s a lot easier for people generally to admit wrongdoing when they’re not being “assaulted” for it. In a sense, you’re joining them, showing that you can understand where they’re coming from and what might have made their questionable behavior irresistible. But at least implicitly, you’re also making it known that you don’t see the situation as they do.