Who actually started the extermination program? Who knew what was going to happen and could have stopped it in its beginning? What individuals bear the most responsibility for the creation and continuation of the purge of undesirables?
This answer to this question is a bit complicated, since "responsibility" can be interpreted in different way, but the participants in the Wansee Conference, along with Hitler's inner circle (Bormann, Himmler, etc.), would be a good place to start.