Should I Kill Adella the Nun?

If you accept Arianna's blood more than 3 times while Adella is in the church, just kill the nun after she gives you the gesture to be safe because glitches have been reported with this NPC, she no longer contributes and eventually goes mad and attacks the hunter for being "impure" anyways.

What happens if I kill Adella the nun?

When the Blood Moon rises (after defeating Rom), and only if she has killed Arianna, Adella can be found roaming the area on the left, just outside the chapel. She will attack the player slowly, stumbling madly, with a dagger. Killing her is the only option from this point. She will drop the Oedon Writhe rune.

Should I take Adella's blood?

The amount of HP Blood of Adella restores is very small and shouldn't be used in emergencies, however the gradual HP restoration is more than strong enough to be useful. Due to the buff to health recovery, it is impossible to use this item while a Beast Blood Pellet is active and vice versa.