I also don't think that Admiral Marcus was a psychopath. I think that he had become an extremist after decades of Cold War between the Klingons and the Federation. He believed that war was inevitable and wanted to fight it now, while he had Khan's advanced weaponry to give him an advantage. He genuinely believe that he was doing something that would ultimately help the Federation, so doing some questionable things and sacrificing some lives was a small price to pay. He was an "ends justify the means" kind of thinker. Being a huge Military History buff, I fully agree that Marcus reminded me of General Lemay later on in his career.