Don't know much about J Buchanan. Why would some consider him the worst?
Buchanan was President as secession was unfolding; he believed that secession was unconstitutional but there was nothing the Executive could do about it. So, that's what he did: nothing. He waited for the Lincoln Administration to take office and thus passed the buck on resupplying Ft. Sumter. He never understood (like Carter) that the job of the President was to lead.
Nor I.
"Why would some consider him the worst?"
Maybe as a diversion to take the heat off Wilson, Roosevelt, and Clinton.