Just kill her. Maybe shatter her soul into the maw?
Her ending feels more like the one arthas should of had. His hard choices actually were to save people before he was made the lichking. Slyvannas's motives ranged from weird revenge plots on people she didn't know, trusting a dread lord, then cowardly killing mostly her own people because she was afraid of judgement in the afterlife...
I can't really understand why she was kept in the spot light but brutally killing her would of worked.
Also no idea why kel'thuzard and arthas had such shit stories.