Scourging alone was lethal.
Crucifixion was a study in horror from the beginning to the end. One of the most painful and potentially lethal parts of the process, however, took place before a man even saw his crossbeam. Before being crucified, a man would be scourged, often translated in the Bible using the gentler word “flogged.” Flogging, however, implies that a person was hit with a traditional whip or a cat of nine tails. These were, of course, horrifically painful, but nothing can compete with the Roman flagrum.
When a man was scourged, two Roman soldiers would alternate blows using Roman flagrums, multi-tailed whips that were filled with bits of sharp bone, glass and metal to tear flesh while lead weights made the leather hit harder and faster. The flagrum would rend flesh down to the muscle and bone. Blood loss was severe enough to be potentially dangerous, and the experience was so painful that it was literally capable of killing a person. Assuming the man did not succumb to pain or blood loss, the flagrum would damage the lungs and kidneys, a death sentence even if the man were not already condemned. When the man was paraded through the streets and nailed to the cross, his back would not look like anything human.
The scourging was meant to bring further pain and horror to the crucifixion. As such, it was not meant to usually be lethal in its own right, at least not when combined with a crucifixion. Roman soldiers were, for the most part, very good at bringing a man to the edge of death without actually killing him. That said, the flagrum was such a brutal instrument that when the soldiers slipped, the whip could wrap around the condemned man’s body and tear at his flesh with such force that he would be literally disemboweled.