Sorcerers knew this, but most of them had ethical lines they wouldn't cross; it was a rare sorcerer who would shed their own blood in willing sacrifice for a cause, or at least a reason, and mercifully rarer still that there were mages without those ethical lines and merrily explored the possibility of power that came through the shedding of unwilling blood.
He was not repulsed by blood, but nor did the field of study have much in the way of interest for him. At least until he scented blood in the air in the form the colored the streak of magic sickly green-red, the feverish pulse of it. Then he heard, distantly, through the magic, a young voice yelping.
Someone had hurt his mageling.
There were ways to speed steps. He didn't think he used them, but he couldn't say for sure, no more than he could say how he found his way through the turning hallways and corridors. The prison block was a channel house of blood and fear and pain, and there were ways to use those emotions; he let them enter him, and transmuted them into wrath.
And turned wrath into punishment.
His hand flex, clenched, spread his fingers, and when he clenched his hand, the guard who was shooting was lifted off the ground and thrown aside. He wasn't the point. Stephen stepped into the room, still with the copper of blood bright in his mouth, and turned dead eyes on the man who cowered by the wall, gushing blood.
But it wasn't enough. His apprentice, his mageling, Loki was trembling and fragile, and there was blood on him, too. It would never be enough. There was power in blood, and knowledge in how to use it. He twisted his hand, and all the power in the blood started glowing, an eerie red mist in the room, as he reached into that brain, and with a stroke, burned it out - and he knew how to do so slowly.
Whitehall gurgled as he tipped to the side, and Stephen dropped his hand; the body slumped further, and he should care but he couldn't. "Loki," he said, hoarser than he wished, full of emotion. The Cloak detached itself from his shoulders, from the pulsing, glowing Eye of Agamotto, and flowed over to wrap around Loki.