[Massive Main Quest Spoilers] I'm confused about the ending

I played almost through to the ending, including the final battle and the tower and everything but it said I failed a quest. I wanted to do it so I reloaded an earlier save and did the quest. When I redid the final battle I got a radically different result. The first time Ciri died, and the second time she lived. Did I accidentally get the New Game+ ending, or does assassinating Radovid somehow save her, or was it something I did in the battle? I guess it went faster but it went about the same.