How is there an us vs them mentality in the Chrysalids?

There is that mentality with everyone. The people who are fighting against mutation think that mutation will kill them and their society, in many cases with good reason, but they have taken that core fight too far... but they don't know that it is too far. They are without the science to know how mutation works or how it can corrupt their community.

The Fringe people are being killed by mutation... not their own mutations, but they can't survive on their own crops. Additionally, almost all of them have been abused by the mutation-fighters, so they are against them... and they have to raid them to survive.

The telepaths, who have the least reason to be against anyone since they can mostly hide, find themselves at odds with both groups, who want different things from them, and then when they meet the people from Zealand, they are also us against them, for some good reasons (people are trying to kill them), but also for some very bad ones (they figure they are better than others and the ultimate in evolution, and everyone else are hopeless savages). And they join them. It is good for them, but their new group isn't really better in terms of "mentality" ... they definitely do not promote unity or peace. :)