Perhaps because most of us suspect that both Hillary Clinton and Donald Trump would make even worse presidents than Barack Obama has been, this has led some people to lose sight of any remotely humanitarian standard and conclude that Obama has been a good president by some reasonable measure.
I strongly disagree, and for the following many reasons.
The Obama administration has sold $60 billion of weapons to Saudi Arabia, one of the worst human rights abusers in the world. We know they use their money and weapons to fund terrorism but Obama gives this stuff to them anyway.
Obama personally ordered a drone strike that killed a 16-year-old American boy born in Colorado. (They claim it was an accident, but this is the non-terrorist son of an actual terrorist that was killed 2 weeks earlier, so I don't believe that.) Obama has assassinated at least 4 US citizens in drone strikes, at least 1 of which was done deliberately. This is the clearest imaginable example of a lack of due process. Colbert entertainingly covered this issue in this video.
Relatedly, Obama has killed thousands of innocent people in drone strikes. As covered in "The Assassination Complex", about 90% of people killed in drone strikes were not the intended target, and even those targeted have not received due process. That is, they are not given the chance to prove their innocence in a court of law before being assassinated; they're just suspects.
The Obama administration has prosecuted more than 3 times as many journalists and whistleblowers under the Espionage Act than all previous presidents combined, arguing that accepting classified information from a whistleblower makes the whistleblower and the journalist "criminal co-conspirators". This is an attack on press freedom, government transparency, and therefore our ability to hold our government accountable.
Obama invaded and virtually destroyed Libya. There was no official declaration of war, therefore this action is a war crime according to international law. Hillary Clinton led this charge, as detailed by the New York Times here.
So in summary, Barack Obama is personally responsible for many innocent deaths and a massive erosion of basic civil liberties. And he didn't do many of the other things he said he would do, like fully get out of Iraq and Afghanistan quickly, close Guantanamo, etc etc etc etc etc.
He's not the worst president ever, but he's been really, really, really bad for all of the above reasons and more.