The fifth and by far the most important reason is that...

The fifth and by far the most important reason is that stoning is literally a means of crushing the murderer's head by means of a rock, which is symbolic of God. This is analogous to the crushing of the head of the serpent in Genesis 3:15. This symbolism testifies to the final victory of God over all the hosts of Satan. Stoning is therefore integral to the commandment against murder.