Strength is a very complex thing, and there are many forms of strength – that’s one of the lessons from Bleach.
Hallibel is a “patron saint” of the phase of life when one struggles with Sacrifice. She is too honorable and too compassionate to sacrifice others. She does not want to sacrifice herself. But she cannot make progress and defend oneself and others without sacrifice… So she yearns for a world without sacrifice altogether. I think that under Aizen she comes to accept VOLUNTARY sacrifice of others for her sake. And I suppose she is able to voluntarily sacrifice herself . But her dream is still unrealistic.
Note that she is not defeated in battle, only by Aizen, whose role is to shatter illusions. BUT, one of the things that Aizen misses (and Ichigo does possess) is the power of self-sacrifice. This is for two reasons:
Aizen is lonely – has no one who would complement his being to sacrifice for
Aizen wants divinity (even if it is not a philosophical absolute, but perpetually surpassing others and perpetually reaching higher…). And this seems inconsistent with rejecting a part of one’s strength, with self-sacrifice, with needing others.
So, Aizen’s killing Hallibel may symbolize his rejection of self-sacrifice. And perhaps his killing Ichimaru symbolizes the two reasons for that rejection.