Power has nature and that is what causes black magic corruption. Harry was already helping Molly to change her nature away from black magic for some time and it clearly had become weaker in those years. The power of the mantle is much greater than the remnants of the black magic in her, it will have much more influence on her. Unlike Maeve Molly has already accepted the responsibility that comes with the mantle. She wants to be a dutiful winter lady. Most of her power and the whole winter mantle wants her to be a good winter lady. All the sense of responsibility steers her in that direction.
She found purpose and that is much stronger than warlock Molly. The danger for her soul is not in black magic. She found harmony with her magic that no mortal can achieve. That is why she can use a cellphone now, no warlock can do so.
Warlock Molly is finished. It is now Winter Lady Molly. Different story.
I'd argue that (like Black Magic) the Winter Lady mantle is fundamentally incompatible with much that we consider "humanity." Mab was once human, too; she pretty thoroughly ISN'T anymore. Molly is already beginning to drift that way...
She has a bunch of new power -- Winter Lady power -- that she didn't have when she was just Molly. But she still DOES (afaik) have those same powers that lured her toward Black Magic; she is still susceptible to many of the same temptations and subject to having her own soul corrupted. I'd argue, in fact, that Winter is liable to push her even HARDER that way.
She is beyond the reach of the White Council's laws, it's true. But the Dresdenverse has "Laws of Magic" that are more like "laws of nature," and
those still apply. By those laws, Molly is still at risk. I don't think "cackling villainy" is the risk anymore; rather, as her soul slips, SHE will be slipping more quickly into pure-WinterLady-ism, less Molly and more Sociopathic-Harlot-Virgin Maeve.
As Bob put it: Welcome to the new Winter Lady, same as the old Winter Lady. The Mantle will come to define who she is, how she acts... much more than anything else does (including "being Molly Carpenter" or "Having been Harry's apprentice").