There are two answers. The first is that being president of the United States involves making moral compromises, even on the best day. Throw in the ambition it takes to attain the office, and nobody comes out of the process unmuddied. Especially since the end of World War II, the United States has essentially been an imperial power — possessing both the ability and the will to reach around the world to affect events and attempt to bend history in its favor. It doesn’t always work out the way we’d imagined: In Vietnam, Iraq, and numerous other countries, we’ve helped create disasters that killed hundreds of thousands of people and harmed the lives of countless others. As president in these circumstances, it’s hard to be a hero when you’re the most powerful human on Earth and yet you muck it up so badly and so frequently.
Even the best-intentioned presidents go awry. After George W. Bush’s failures, Barack Obama came into office promising a more modest approach to the world. He ended up ordering drone attacks that killed civilians, oversaw the assassination of an American abroad, used mass deportation to try to bargain immigration policy with Republicans, and was in charge of the National Security Agency apparatus that vacuums up huge amounts of personal data from people all over the world. Obama is a hero to many American liberals, but for many others still, his presidency was a horror show.