Kyubey. He pushes the envelope by being responsible for botching Kyousuke's operation, and thus having Sayaka contract with him at that time instead of during Walpurgis Night where she would have been effective. He also was the one who planted Charlotte's Grief Seed at the hospital in an attempt on Mami's life and thus have Madoka break down and be forced to contract.
Ryuzaki, who allies with Kyubey and helps himkill Kyousuke, and thus turn Sayaka into a Witch and have Mami and Kyouko kill themselves to destroy her. All in the purpose to have Madoka become a Goddess. And his plan works.