Long before its adoption by Christianity, the cross was a symbol of the four-fold world and its various measures: Earth, Air, Fire, Water; North, South, East, West; Birth, Life, Death, Afterlife.
With its inherrent link to the number four, the cross is also related to the swastika which, before its corruption by the Nazi party, was a powerful symbol of the Wheel of Fire, or energy in motion.
To many mystics the cross represents sex differentation, with the male (upright) and female (downright) forces emanating from a balanced point (horizontal). The Romans, specifically, used the cross to represent reproduction. To be put to death by crucifixion thus came to represent that the person being executed should never have been born in the first place.