Of course I'll deny you. And I don't need to assume. I just know you aren't the kind to make obvious mistakes. The first epitaph requires two people. The key who chooses the sacrifices and the one who kills them for the key. Either you are told to kill these people by someone, or you never touched them yourself, be it magic or otherwise. And the second line says it's the survivors, which means it has to be someone in the family. I can't imagine a 'magical ritual' would allow for such errors.

[He looks around, taking a good look at the scene.] So it's not a locked room murder. That means the murderer could do this, at least with what's known so far. The symbol...well, I have to check, but I think I might have seen it somewhere. I'll have to look it up before I say what it is.