Mab wasn’t going to kill Molly, only the White Council were interested in killing her as a Warlock.
To be blunt, Her abduction was saving her from the known attention of the White Council, in hindsight Harry was warned of a Warlock in Chicago by the Gatekeeper who we only subsequently found out was in league with Mab. The Gatekeeper surveyed the possible futures and nudged events towards the desired future of Harry apprenticed to Molly (see the Molly soul Gaze)
The Gatekeeper senses a Warlock, is prescient about her role in future events, alerts Harry and ropes in Mab to keep her safe by abducting her with indication of a new Winter Lady candidate, until Harry can collect her and take her to the White Council where their co-conspirator Uriel has arranged Michael to come out as the saviour of the Baby Wizards, just when they are ready to execute Molly.
It’s the Gatekeeper manipulating events so Harry gains Molly as an apprentice, who will eventually replace Maeve as Winter Lady, which is also vital to both Mab and Uriel’s future plans. There were absolutely no coincidences.
In tandem to this you have Crane/Madrigal setting up Splattercon as his own buffet, and opening the backdoor to Arctis Tor, the Convention and attacks would firm up the link to the Fetches abode in the Never Never in Pell’s theatre, allowing the Black Council to use it to ‘rescue’ Lea/ survey the Backdoor to Demonreach in Mab’s Ice Dungeon. We have to presume Mab was aware of an attack by the Black Council, what surprised her was the Denarian involvement.
It’s clear The Gatekeeper, Mab and Uriel (Plus Vadderung and Hades) have been working together throughout the entirety of the Files. We only really see this from the end of Ghost Story onwards.