With Baiyi's current level, he was still unable to cast any forbidden spell like Instant Death, let alone making a scroll with it. In truth, not all forbidden spells could be cast just by using incantation like the Harlek’s Incantation of Incineration, most of them actually needed various magical materials, tools and even formations to aid the casting, some were even affected by geographical conditions and weather constraints.
That was also why the Book of Servitude of the Sage-Emperor of the Magi was classified as a Divine Weapon. Because the item allowed the caster to be exempted from all of the constraints of the casting conditions, the Archmage was able to readily cast any magic that he liked, playing the domineering tyrant and dominating everything with the help of the mighty item. Oh, you don't like my way? It doesn't matter. I'll drop two forbidden spells and see how you like it now?