To unlawfully, and inhumanely, physically kill another. This is the ultimate form of violence that people enact.
Capital punishment does not solve the problem, for another murder does not solve any situation. To send into the world of spirit souls that are unready, merely increases problems and does not lessen them in either world.
Whatever redress is performed, it must be an attempt to help and not exact revenge. All punishment must be remedial and redemptive. The object should always be to try and enable the soul to come into its own.
In spiritual terms, many suggest that there is no such thing as murder--for it is a physical act in which consciousness continues. Some contend that no one is slain who does not want to die, for just as the slayer picks out the victim, the victim seeks out the slayer.
An individual who kills will ultimately learn what killing is by being the victim. There is no salvation or peace when violent acts are committed. In broader terms, mankind must learn that not only is human life sacred, but all life--including that which we cannot visually see with our eyes.