Simply put, Ella confessed because she is better than the church. Sawing off a living person's arm certainly made her rethink the way she does things, and if nothing else, she doesn't want to fall down to the church's level and have her opponents drugged and rid of any human decency. Moreover, she has to beat all the other sisters to get the ring anyways, so if she dies like this, it just means she'd have failed anyways.
Or at least that's my current take on it; I haven't read past volume 4 yet.