I think it'd work more if, instead of a glorified palette swap, Day Breaker came about because Celestia noticed her subjects growing apathy and perhaps even despise of her sister causing her to grow she grew disappointed in them. Soon that disappointment turned to anger, and that anger into hate. That cold hate distilled into a great disgust and antipathy of the ponies she used to love and would have lain her life for. So when she finally snapped and decided to purge them all her sister was forced to stop her.
She still loves her sister, even though she wouldn't let Day Breaker do this for her. But not to worry, when she returns she'll save her sister from the torment the ponies must have put her through in the thousand years she was away.
Basically, Celestia goes Yandere when she becomes Day Breaker. This is my interpretation and I'm sticking to it.