Why do we all assume the spell was the Red Court's, and that it was a prized possession they've used before? The impression I got was that the Red Court was given that spell, just like Victor Sells was, and that they were HORRIBLY BAD AT IT. They had to sacrifice how many people, to get enough juice to kill three wizards, and maybe a few people on Susan's side of the tree? Say Susan's parents and grandparents were both alive, and she had two siblings- that's twelve people; we'll say fifteen to be safe. Victor managed to kill two completely unrelated people at the same time without any sacrifices. I'm not saying that there's no problem to go from killing two unrelated people with one spell to killing fifteen related people, but to be fair Victor Sells wasn't really a wizard, but a gifted person that had learned a few tricks. The Red Court had multiple sorcerers working on the spell, had props to go along with and beef it up, and while they might have needed some sacrifices even still... I am not too impressed that the Red Court needed that much to pull it off (they were killing for hours while Dresden was on the way there). Granted, Victor had a storm to beef his spell, but they had a ley line running right underneath.
I don't think it was there spell; I think somebody as yet still unrevealed was manipulating Victor and the Red Court.
Or ya know, they knew
way more about the Blackstaff and how it interacts with it's user than we do*. Or, Eb even without the Blackstaff. I can very well see it taking a magical nuke to get at the WC wetworks man, not just a howitzer.
*Don't forget it's stolen, and the real owner wants it back.