User blog comment:Ncduru/Who is Anna?/@comment-4999650-20151211175244/@comment-7418318-20151211222604

Originally the Fiore Royals planned to use some other means to open the Eclipse Gate and only changed plans when they found out that the entire Zodiac set was present. And with the way Arcadios talked about "sacrificing" the Celestial Spirit Mage, he apparently expected the one to open the gate to die.

Combine that with what we recently learned about Layla's death and I think it paints a pretty clear picture about her death, especially if you further add her weak constitution into the mix.