I think a key to the SBTC notation might well be--is this a phrase he's used fairly often? Did he make similar comments when signing other yearbooks?
That would be interesting to know if he did it more than once. But if he only did it ONCE, to me that's very odd to put on a HS yearbook signing page (SHALL BE THE CONQUEROR), and then to have those exact letters show up as the signature (S.B.T.C.) on the "ransom note" in a murder case to which he later confesses...