User blog comment:Xilinoc/Ch. 567 (Rukia be like "I ain't even scared though")/@comment-5693502-20140205150657/@comment-5693502-20140205230723

Well yes biologically she could be seen as "dead" but her mind is still functioning. Your body doesn't fear death, your mind fears death. Sentience fears death. A comatose person won't fear death but they're still alive. Gah, I'm reading too much into it again...

I guess Rukia might have shut down the part of her brain that processes emotions, that would definitely make her impervious to fear (and any other emotion for that matter).