The law not only prohibits certain negative behaviors and attitudes, but by implication it requires certain positive behaviors and attitudes. That is, if adultery is prohibited, chastity and purity are required.
When we apply these patterns set forth by Jesus to the prohibition against murder, we understand clearly that, on the one hand, we are to refrain from all things contained in the broad definition of murder, but on the other hand, we are positively commanded to work to save, improve, and care for life. We are to avoid murder in all of its ramifications and, at the same time, do all that we can to promote life.
Just as Jesus considered lust a part of adultery, so He viewed unjustifiable anger and slander as parts of murder. As lust is adultery of the heart, so anger and slander are murder of the heart.
By expanding the scope of the Ten Commandments to include such matters as lust and slander, Jesus did not mean that it is just as evil to lust after a person as it is to have unlawful physical intercourse. Likewise, Jesus did not say that slander is just as evil as murder. What He did say is that the law against murder includes a law against anything that involves injuring a fellow human unjustly.
How does all of this apply to the abortion issue? In Jesus’ teaching we see another strong reinforcement of the sanctity of life. Murder of the heart, such as slander, may be described as “potential” murder. It is potential murder because, as an example, anger and slander have the potential to lead to the full act of physical murder. Of course, they do not always lead to that outcome. Anger and slander are prohibited, not so much because of what else they may lead to, but because of the actual harm they do to the quality of life.
R.C. Sproul, Abortion: A Rational Look at An Emotional Issue