[spoiler]While he did not do in Mami, he did push Sayaka and Kyoko both to die faster, since he wanted Madoka-who-has-all-the-power-he-needs to contract.
Sayaka's Case:
Root causes
- Heroic Personality. When stressed, she tends to go all out and hasten corruption via magic depletion. It's a pretty deep part of her, given that what we've seen of her in Oriko has her battling familiars WITHOUT any knowledge or power, just a candy cane and a ribbon-stick ripped from the Barrier itself.
- Ms NTR. She likes Kamijou, but can't spit it out, and Hitomi will nab him, causing a classic teenage girl breakdown.
- As an extension of her paladin complex, she will reject help and the methods of other less than heroic sorts.
Kyubey's nudge: Telling her how she can be more heroic and also rubbing in the pretty lich aspect(that is an awesome phrasing btw). The 'disregard pain, let regeneration handle it all' method taxes her magical reserves, not to mention the superspeed enhancement she started using.
What is different:
- This time she would go into it(if she does indeed contract), knowing the dangers and at least the consequence of magical depletion. Due to her heroic personality(much like Mami), she would strive to avoid Witchdom, at least. Possibly she might kill herself before she corrupts all the way.
- She'd have support. While she does not trust Kyoko(ruthless facade) and Homura(ruthless for anything not-Madoka), she does trust Mami(heroic type just like her), and probably Yuma(shes cute, who can resist?). This'd help avoid fatal isolation.
- Madoka is dead. It'd be more profitable for the girls to defeat Walpurgis, collect her grief seeds and then inevitably corrupt. They taste sweeter that way, and it helps if you don't kill the goose that lays the golden eggs.
EDIT: - ALSO, Hitomi owes Sayaka her life. Knowingly so, unless they got memory wiped. She probably wouldn't yoink her savior's boyfriend, would she?
Kyouko's Case:
Root cause: Frustration mostly.
Kyubey's Nudge - Nudge, nudge, hint, hint, maybe Witch can be saved. This is a totally unnecessary push, except he wanted to force Madoka into contracting.
What's different: Madoka is dead, theres no reason to nudge her. Better to wait for the natural end.
[/spoiler]