Mai was killing herself without actually knowing that's exactly what she had been while killing the 'demons'. She never knew that she was killing herself off, but subconsciously, she was fighting against what didn't make her fit in. In the flashback, she had come to the false conclusion that her powers had driven off Yuuichi, and that is what started her subconscious exorcism of her own powers.Originally Posted by shinta|hikari
In that manner, she was figuratively "killing herself" (hence all the bruises she had developed) in order to be able to make friends and fit in. She just didn't realize the side effects. That's what made Mai's arc so tragic. Fortunately though, it could end happily after she forgave herself.
I'll admit I wrote it somewhat ambiguously before.