The problem, as I see it, is that individuals may not be "rotten to the core", but movements can be. The most active participants are often the worst ones, and the less-awful ones will often make excuses for them. Even by saying nothing, but simply ignoring it while identifying with the group, they will be seen by both enemies and allies as approving of the worst behavior.
In my opinion, the best way to "get out of this mode of thinking" is for people to look at their allies and say, "Hey, can we tone it down a little?" As it is, right now, it's the ones who are not "rotten to the core" with the most ability to change things. And failing to do so means that they are, indeed, "systemically undermining good in the world".
Unfortunately, asking people to tone it down is a dangerous tactic. It may improve the world, but at a cost of decreasing your own power. That's why the nastiness not only persists, but expands.