The Bible is not specific on the subject of abortion. Many pro-abortionists take this as tacit permission for abortion. If that were the correct way of handling scripture, however, then one could argue that pedophilia is permissible because the Bible does not address it specifically. Why does the Bible not mention abortion? It may stem from the fact that the very idea of abortion was contrary to the ancient Jewish view of having children. Children were seen as a blessing. In fact, for a woman to be barren was often considered a curse for God and something to be lamented.
The Bible does have verses about killing innocent people. The most obvious is Exodus 20:13: "Thou shalt not kill." So what makes murder wrong? The more common response to that question is that it is a societal norm or societal tradition. That response, however, implies that there could be a time when murder is OK. Or even racial cleansing as Hitler attempted in the early part of the last century. The concept of "where do morals come from" could be debated, but the Biblical scripture had another reason.
Murder is something done to God. It is an affront on His image. This is affirmed in Gen 9:6: "Whoever sheds the blood of man, by man his blood will be shed, for God made man in His image." This gives man a unique position in comparison to all of the other created creatures. There are no Biblical restrictions on killing animals in any general sense. Because of man's privileged position he may not kill any other man. To do so is to desecrate the image of God. God's image, or man's person hood, is what separates man from all the other creatures on planet earth. We are bearers of God's image and are obligated to respect that image.