Darkness and Lust don't seem inherently evil to me, but assassins and succubi are (in D&D at least- I might think of assassins as neutral a la Terry Pratchett).
How about Nocticula has to renounce or reform her darker worshippers. Destroy powerful Harems/ take over Assassins guild (without using her power over them?)
How might she become a god of other groups? Perform miracles or great feats?
I hope you and your player have a good time!
