1. Mab miscalculated and Slate is not as strong as she expected.
But is this true? I mean Sarissa states to Harry that Slade didn't begin as the weak thug/monster that he knew. So can it all be blamed on the effects of the mantle? Or were other forces at work here? Aurora was mad and talked Slade into murdering her own Knight. How did this come about? What persuaded Slade to even conspire against his Queen, Mab? I do not believe these events all happened in a vacuum.
Lea got infected by way of the knife,gotten at that infamous party that a lot of other supernatural powers were in attendance. Contact with her and or the knife infected Maeve. We have the word of Mab on that. Contact with either could have infected Slade, he is a mortal so the effects of Nemesis might work quicker in him. However that still doesn't explain Aurora's madness. Did she just go insane? Or was she also infected by Nemesis? If so, how? I submit that Nemesis was indeed behind it.
Who stood to gain if Mab went down? And in her place a spoiled spiteful child to paraphrase Mother Summer replaces her... Who is going to command Winter's Army against the Outsiders and guard the Outer Gates if Mab goes down? An infected, spoiled, spiteful child, Maeve.
Summer is there to keep Winter in check, but not to defeat her.. So what were Aurora's motives for wanting Mab brought down? Who stood to gain by that? Outsiders, Nemesis.. So it stands to reason that Aurora was infected, she in turn came in contact with Slade and brought him into her conspiracy to over throw Mab.. How did that come about? Did Slade routinely come in contact with the Summer Court? If not, who ordered him there? If it wasn't Mab, it had to have been Maeve.. Maeve, who may very well have been infected at the time.. If she sweetened the deal with sex, in turn infected Slade.. Slade coming in contact with Aurora, in turn infected her. She being mad, and Slade being mad, then conspired to kill her Knight and frame Mab for it. Maybe that was the bigger game that Mab finally figured out, why she decided to kill her daughter rather than try to cure her like she did Lea? Or was Maeve just too far gone?