I think self-delusion is one of the most unique 'superpowers' that humans have. We can believe anything, be absolutely 100% certain of anything, no matter how contrary it may be to all the input of our senses and all reasoning of our minds. A whole lot of it boils down to "This thing is completely morally wrong, and people who do it are bad people. But I do it and I'm not a bad person."

And this sort of cognitive dissonance is really painful. The natural reaction to being confronted with it is to just go "nuh-uh" and stop thinking about it, or to give contradictory circular justifications which you can avoid thinking through in order to satisfy the desire to square contradictory views without actually doing so.

(Common example, imagine believing simultaneously that dogs have rights and should be treated with kindness and that other animals which are very similar to dogs can ethically be farmed for their body parts. Completely contradictory... and extremely common, even here)