Because he was being held in a high-security prison guarded by Ephidel and Jaffar, soon to be Nergal, and the huge Black Fang force that was supposed to stop Eliwood & Co.? Plus, the ritual was coming up.
Nergal is hubristic. As he states explicitly, he wants to see Eliwood & Co. in a state of abject fear, desperation, and failure. Common antagonistic trope.
If this is a flaw, then every RPG in existence is equally flawed. Why can Cloud take bullets and receive 5 damage in FFVII, but Aeris can't take swords? Why do units in Fire Emblem survive after being impaled by spears and having their heads twain by fighters?