Adella will kill Arianna if you choose the Arianna's tainted blood over her own, possibly out of jealousy. ... You can't obtain her gesture or her blood if you haven't talked to her in Oedon Chapel until the Blood Moon.

How do I stop Adella from killing Arianna?

To stop Adella from killing Arianna, simply remove Arianna's Blood Vial from your inventory and instead take Adella's Blood. If you defeat Rom with Adella's Blood in your inventory, Adella will not kill Arianna.

How do you keep Arianna alive?

First kill the three bosses and don't speak to her and then kill Micolash, Host of the Nightmare. Then speak to her and send her to the chapel where she'll be found sitting, she is alive even if the player has the other One Third of Umbilical Cords near the end of the game.