The complex reailty of humans is that both viewpoints are necessarily true at the same time.

It is unequivocal, scientific fact that various propaganda, de-humanization, etc. techniques work on a statistically verifiable scale, and a huge portion of a populace can be radicalized/directed with very simple techniques applied effectively.

It is also morally imperative that we never remove individual responsibility from the final, day to day choices and actions made by a specific human (barring actual medical mental unfitness).

Both viewpoints are true, and must remain true, at the same time. Yes, you can brainwash people with statistical relevance. No, being brainwashed is never a justifiable excuse for doing horrible things to other people.