He is saying that god already knew the person that the fetus is going to become; therefore the fetus does have a pre existing life. Not only does that life pre exist, but the fetus is said to become a prophet to the nations. By killing that future child, you would be killing a life that could affect thousands in a positive way.
1 Corinthians 3:16-17 says, "Do you not know that you are God's temple and that God's Spirit dwells in you? If anyone destroys God's temple, God will destroy him. For God's temple is holy, and you are that temple." God makes very few threats, as you may have seen while reading the bible. But here, it says not only are you destroying the fetus, but you are destroying the spirit of god that dwindles within the life and very being of the future child. Each and every one of us, regardless of our religious views, has the spirit of the lord within us. I believe that whole-heartedly. Therefore, if I murder a person who is atheist, it is the same sin as murdering a preacher, or a fetus, because not only are the people being murdered, but also the spirit of god.
St. John says, "This is the message you have heard from the beginning: we should love one another, unlike Cain who belonged to the evil one and slaughtered his brother" (1 John 3:11-12). As I said before, slaughtering your brother and a life that has not yet seen the day is the same sin. You are still murdering. Cain was said to be EVIL because he slaughtered his brother! If you are evil, you do not love, and that is a sin for it goes against god's word which says, "Here is my command, love each other." To fail to help people in need and or in danger, especially those who cannot defend themselves is failing to love. In the human life, there is no more dangerous place, than the womb of your mother. There, anyone or anything could do whatever it wanted, and you could do absolutely nothing to defend yourself.