"If abortion is murder, why do some Christians defend it?"
People - Christian or otherwise - who doubt abortion is murder and try to defend it usually do so by redefining murder exceptionally narrowly and/or by claiming that these Bible verses about abortion apply to babies whom they inexplicably declare to be the exceptions to the rule (and self- declaration doesn't necessarily prove a person Christian).
The question of whether or not abortion is murder typically leads to asking "when" is abortion murder, which in turn eventually leads to asking what is a baby and what isn't yet a baby, but merely a zygote, an embryo or a fetus.
You most likely have good sense of judgment and perception, so instead of letting other people draw conclusions for you, draw your own conclusion on whether or not abortion is murder by using this quick and simple test, as follows:
Step #1: Please look carefully at the 14 photos below and count how many of them show babies and how many of them do not show babies. Here is the first photo: