The ransom note is SIGNED with "Victory" and then, "S.B.T.C".
I'm not buying the Shall be the Conqueror either. Where's the identifier? Who is the conqueror? Where is the "I"? If it was really so important, he would have spelled it out so there would be no question in anyone's mind who held the power. He would have signed it more along the lines of - "Victory! I AM the Conqueror". SBTC in the note is meaningless and is of no consequence the way it is written to anyone but the writer.
On the other hand it could be the writer wanted to mislead the police and wrote the letters off the cuff.
On the third hand (or would we be to feet now?), it could be a common greeting or saying. I'm not leaning toward a cult, but as an example a cult/group/secret society might end conversations with "So Be The Cause" or "So Be The ... whatever".