Their life is taken without thought to compassion, sympathy or morality. The basic, underlying reason why someone murders innocent people is selfishness. It's rooted in the need to provide a way to satisfy the murderer's needs and desires, not the needs of the murdered or of society. There are several reasons. One is because of rape.
Rape is an illegal, violent act that violates the laws of society.
If a woman becomes pregnant because of that illegal act, the child being developed within her has been created illegally. The child she is carrying was not created by any decision on her part to willingly participate in the formation of life. As such, she is under no legal obligation to allow that which is unlawfully growing within her to continue its growth. This is consistent with the laws which society has placed on individuals. For example, if someone robs a bank or acquires money illegally, and then buys something with that unlawful money, the items which they've purchased are not legally theirs and can be taken from them.