Abortion is murder, therefore it should be illegal.

My claim is "Abortion is murder, therefore it should be illegal." 

Let me first start by giving the definition of murder as described from the Merriam-Webster English dictionary.

Murder : "The crime of unlawfully and unjustifiably killing a person." 

Now let me give the definition of abortion as described from the Merriam-Webster English dictionary.

Abortion : "The termination of a pregnancy after, accompanied by, resulting in, or closely followed by the death of embryo or fetus.

Now, a person is described generally to be the body of a human-being. Therefore, an unjustified killing of a human-being, is the definition of murder. 

Both an embryo and a fetus contain human DNA at all stages throughout their development. 

Now, what makes a human a human? Well, generally humans are considered humans not based on their development as a person, but rather their genes. These genes, come directly from Deoxyribonucleic Acid (DNA). Since your DNA is formed at conception, this means you are a human at conception. Which therefore means that abortion is the murder of a human.

DNA is  formed at conception. 

DNA makes a human, a human.

Murder is the unjust killing of a human. 

Therefore, by definition, abortion is murder. 

Murder is illegal, therefore abortion should be illegal since it fits under the criteria of murder.

Using the terms "abortion" and "murder" requires assumptions to be made. 

First, the word abortion means the termination of a pregnancy before the fetus can survive outside the uterus.  It can occur spontaneously, known as a miscarriage, or be induced intentionally through medical or surgical procedures.  I have to assume you're not referring to spontaneous abortion, but I don't know that.  For example, do you include spontaneous abortion (miscarriage) that may have been caused by the actions of the mother or another person?  I also don't want to assume that you're referring to a D&E procedure recommend by a physician because the fetus will never be viable, is endangering the life of the mother, or the heart has stopped, for example.  

The word "murder" implies the premeditated, unlawful killing of one human being by another.  I can definitively say that not all abortion is murder simply because spontaneous abortion, for example, is not premeditated and most often (but not always) lacks an intent to end the pregnancy.  

It's difficult to substantiate the argument that all abortion is murder without defining the terms more specifically and also making an argument for when life begins and under what construct.  For example, under a religious construct, one might argue that life begins at conception or that life begins upon the moment a baby takes its first breath.  I've heard good arguments for both confined to a religious context.  If you're arguing when life begins in a legal context, you'd just have to open the argument by identifying which law (national, state, etc).  

In summary, I would argue that not all abortion is murder so it should not be illegal in all cases.  First, not all fetuses aborted are viable, some pregnancies threaten the life of mothers who have become pregnant against their will, and many are spontaneously aborted without the ability to show intent on the part of the mother or another person.  If all abortion were deemed illegal including medication, in-clinic, and D&E procedures, the lives of both mother and fetus would be put in undue jeopardy because these procedures are often used to save lives as well as terminate pregnancy.