Angelus had seen the rage and hatred that burned within Xander as clear as day as he fought the youth, just as he and his alter ego had seen it many times before.
There was a darkness inside Xander that he fought against every day of his life to suppress, burying it deep within himself, a darkness that if unleased would cause untold destruction to thousand if not millions.
His choice made Angelus knelt before the fallen form of Xander and sank his fangs into the youth’s neck, draining his life force from his dying body before slitting open his own wrist and allowing his blood to drain into Xander’s mouth, massaging Xander’s throat as he did, forcing Xander to swallow his blood.
You will be one of my greatest creations… Angelus thought to himself as he hosted the prone form of Xander up onto his shoulder before disappearing into the night, unaware of the battle now waging inside of Xander.
When a person is turned the blood ritual of the siring forcibly rips down the barriers that protect that person’s soul forcing it to fight against the demonic entity channelled through the siring vampire from the very pits of hell itself.
The stronger the person’s soul was the more powerful the demonic entity channelled by the ritual and ultimately the stronger the vampire they would become, normally a person with a weak sprit would become a weak vampire, a minion at best while a person with a strong soul would become a master vampire, but there were many classes of minion and master class vampires just as there where many sub species of animals in the world and what Angelus had grossly misjudge was just what kind of strength laid in Xander’s soul.
Even the primal spirit had recognised Xander as an Alpha and even though the primal spirit was later exorcised it left and imprint on Xander’s soul, a form of latent primal power and later Xander would have the discipline of a special forces solider blended into his being from the Halloween incident on top of his impressive personal willpower.
All of this meant that Xander had one extremely strong soul, one of the strongest which would mean next to nothing in reality except when it came to a few things such as being sired.