Ask yourself what you would have done, or what they should have done differently in their situation. Try to put yourself in the mindset of the people you're reading about when you're thinking about judging them. A lot of things people have done in history that we judge as immoral and they as terrible people become a lot more nuanced. It's easy to just dismiss McCarthyism and the Second Red Scare as paranoid, unconstitutional, wrong, but it's valuable to understand that such men were legitimately terrified that a very real and actively hostile power was working to undermine them, and how would you react if you genuinely felt the country you cared for was in danger of being destroyed from the inside?

Not to say such actions were morally right, but it's easy to judge people from the past who made their decisions under pressure and in situations we can't comprehend, and we should try and at least understand why they did what they did before we throw them under the bus.