If the people that work for me screw up, it is ultimately my fault and I pay the actual cost of what happened and not normally the employee. If they cost me more than they produce over the long term I have to replace them. Accidents happen, we are all human. I have to judge how many I'm willing to accept before replacing an employee. It is my decision, my responsibility and I normally bear the end consequences of what an employee does or does not do.
It is no different for a President. And it isn't as if the secretary of defense is some distant job position relative to the Presidency... It is a job directly under the President.
But they’re still the ones that screwed up. When I screw up I don’t turn to my boss and say “you shouldn’t have hired me”, I turn to him and say “I screwed up”. McNamara screwed up, period.