One of the issues is that the US doesn't have much, if any, history and as such still feels the need to heroise the few historical figures it has. When we in the UK look back over our thousand-year sweep, we have fewer qualms in saying that such-and-such a King/Prime Minister/General/ Scholar/Author etc was a psychopath, delusional, a charlatan or a general bastard. Much of our teaching of history is about where these people went wrong, and the absurdity or harmfulness of the things they believed and did (consider Henry VIII and his blood-spattered search for a male heir - a search of which the English Reformation was a mere side-effect). Even as we speak, the legend of Winston Churchill is being torn down as he is revealed as the sexist, racist, snobbish bastard he was. Every day, more and more truth comes out about Thatcher's determination to seize and retain absolute power. But the longer perspective allows for a degree of objectivity and an awareness that while what was done then had a part in creating what we have now, what we have is not necessarily what the people who did those things wanted or intended!
In the US, however, there is still an idea that the Founding Fathers had a specific intention for thier new Republic and that Americans should try to actualise that intention. Which is, of course, untrue.
Just be careful about using the word 'evil'. Because evil is always and only what the other guy does, and using it puts a wall between the people you're talking about and the ones you're talking to. If you say the Founding Fathers were evil, you make them 'other' and then have problems convincing white people that their innate racism and the structural inequality of US society comes from the example and actions of the Founding Fathers. You need instead to demonstrate the line of descent from the brutal, cruel and repugnant actions of these long-ago men, and show how it connects to racist attitudes and actions today. "Everything you believe in comes from a gang of child-rapists and sadists. Are those the people you want to emulate. You're better than that, surely?"
