Taking cues from your character and setting, specifically:
Maybe some old religious or ancient name...
She is a fallen druid...
She loves dead animals...
She supports the idea that death is not the end...
There is a Celtic God Named: Cernunnos
Cernunnos was a Celtic god of fertility, life, animals, wealth, and the underworld.
Celtic (Druid), Life, Animals, and the Underworld (death)! Hits all four!
You can play with the name, make it feminine: Cernunna, Cernunnia
Or create a special type of necromancer: Curnunnian Necromancer
Of course that means writing a bit of background about the God, Curnunnos, to your story so that your readers will understand the meaning behind your term for her, which could be fantastic.
But if you want a term that your readers can understand right away, then I suggest building a new word from two familiar titles or classes of fantasy characters:
Necromancer + Druid = Necrodruid
Necromancer + Shaman (spiritualist / animalist) = Necroshaman
Necromancer + Cleric (healer) = Necrocleric
Necromancer + Paladin (holy knight) = Necropaladin
By the Way, having a dark assumption of the word necromancer could be used to your advantage depending on the story. For instance, if the other Druids call her by this "dark" title "Necro-something", then perhaps she is simply misunderstood and the story reveals her true goodness which shapes the readers' and the other characters' understanding of the title from dark towards light over time.