The Retrospective Prime Directive

Regardless of what we discover, we understand and truly believe that everyone did the best job they could, given what they knew at the time, their skills and abilities, the resources available, and the situation at hand.
-Norm Kerth, Project Retrospectives: A Handbook for Team Reviews

If you do not go with such a mindset - then the retrospective will be painful and a waste of time.
Again , unfortunately if you have sub-par members on the team - they will have done their best and that still is not good enough.