I'm also not an American - but as far as I've read - massive abuse of power in using the FBI to spy on political rivals, illegal wiretapping, illegal surveillance of US congressmen and even presidents, running the FBI while they were doing extremely controversial programs like COINTELPRO and programs and investigations that tried to hinder the civil rights movement, etc.
As a non American, I think COINTELPRO is the single most anti-democratic abuse of power ever done by US government.