It seems harsh to compare pornography with killing. After all, with almost all pornography the person is still alive. Though the person may still alive, with pornography the person is not viewed as a person, but an object. I would contend that anything that lessens a person to an object removes the personhood of the individual and thus kills the person.
Pornography treats all involved as objects to bring others sexual gratification. They are not treated as persons anymore. They are being used. And even though the act has already been done, viewing such material does the same. The persons are treated as objects and are continually being used by others.
Sure, there is the argument that people are willing to submit themselves to being in pornography, so what is the harm in viewing it then? Well, is it right to kill someone who would let you, if you so desired? Mind you, they are not asking to be killed, just that you are given the power to do so, if you will. What about abusing someone who is ok with it? Think of the woman caught in an abusive relationship. Is it ok that she is abused? After all, she subjects herself to it by merely staying in the relationship and not defending herself.
There are many ways other than pornography that persons are treated as objects. In those cases, too, a person is stripped of their personhood and reduced to an object. Once they are an object and no longer a person, what would stop someone from using, abusing, or killing?
A person has a dignity that an object does not. A person was created in the image and likeness of God. A person is an adoptive son or daughter of God. God loves them like we love our children, but even more so, since God is perfect and can love perfectly. Surely, that is what we are striving for: perfection that we can only find in Heaven. After all, we are to love one another as God has loved us!