Bhishma had said before, I will never slay Shikhandin. He was created a female by the Creator, but has became a man out of chance only.
अब्रवीच्च पुरा भीष्मो नाहं हन्यां शिखण्डिनम्। स्त्री ह्येषा विहिता धात्रा दैवाच्च स पुनः पुमान्॥