Ye is right, Hitler was not all bad.
No one is all good, no one is all bad. It’s embarrassing to live in a society where people pretend this is controversial. Only a Sith deals in absolutes.
Thinking that someone is all good or all bad will cloud your thinking. Everyone you love is flawed, everyone you look up to is flawed, and if you try to blind yourself to that, you risk corrupting yourself by rationalising or adopting their flaws, and you also become incapable of doing anything to help them with those flaws. It’s also an unfair and impossible standard to hold someone up to.
Similarly, if you convince yourself that some historical figure is a pure evil caricature, you risk corrupting yourself by opposing everything about them, including the good. And if you believe someone is pure evil you can justify a pure hatred for them, and this interferes with thinking clearly. Without clear thinking, you’re lost.
Pure hatred is also self-corrupting since it’s a rejection of love at a fundamental level.