So much blood.
It had to be done, of course: there’s powerful manna in blood, and even more so if it is spilled by murder. What we are doing will take all the manna from this act and more besides, and leave us gasping and withered and dying.
The children will be safe, and their children too, for at least six generations. By the time our spells fade our descendants will be so great in number as to no longer need our protection.
What are the lives of a deceitful merchant and his callow servants compared to that?