Truman and Papa Bush (the fact that Bush suckered Saddam Hussein into invading and occupying Kuwait and then responding by going on one of the nastiest mass-murder displays in U.S. history and the fact that Truman dropped not one but two hydrogen bombs on noncombatants when virtually every one of his military advisers told him that it wasn't necessary)

Would would be my answer to the question, "So, who are some of the Presidents that you had initially thought were decent but upon closer inspection determined them to be little more than vile, off-the-rack war criminals?