Soushirou's mother dying giving birth to him helped him survive Misa's cooking
In the exposition given by the enemy mages; when the Church Militant had created "Mage-Killers" by feeding them magic-drenched food; it killed most of them. Soushiro was already "modified" at birth when his mother died from mana-loss and giving birth to him; the food completed the process. Ghost-Script Misa was programmed to drag someone down the Abyss.
Hyuang was evil, sure; but was it really necessary for Ghostscript Misa to sacrifice herself to drag him into a Fate Worse than Death? The Vortex was just to contain the Sorceress; Hyuang wouldn't resurrect if he were killed. She could have just shot him in the back with swords; he wasn't paying attention. Misa had to gear herself for the sacrifice enough to ingrain it into her Ghostscript. The Misa Ghostscript was smart enough to realize Koyomi had redeemed Miyuko; but had to take someone.