It’s very easy to convince yourself that you’re doing what you’re doing to serve Hashem. That’s why we all have an obligation to learn Torah on our own, and also to have Rabbonim who have greater Rabbonim they consult with etc. And that’s why we have the Oral and Written Torah which tells us what to do.
Even the most misguided people don’t think of themselves as bad, but as misunderstood.
But I agree with your overall point.