HoV: Hold on. Kagura's plan was to trip Kuroto as soon as he got close to the gate, dunk him in the pool (hand, head, doesn't matter), and offer his (their?) control of Marko's body as the price for Marko to use the gate. At no time did she actually plan to go along with him unless circumstances seriously conspired. It might still have unexpected side effects, or he might wind out of it, of course.
(And in any case, she wouldn't threaten, she would just slash - Kuroto broke the deal, Kagura made it clear what she would do.)