That doesn't explain too much...not to me, anyway.
SCTC could stand for quite a few things.
The strangeness is that Karr made such a big deal and flourish with this in a HS yearbook, then years later confesses to a notorious murder in which not only SBTC appears as a big deal and flourish at the end of the ransom note, but the word Victory accompanies it, which dovetails quite well with the concept he was trying to get across in the yearbook entry.
The idea of personally triumphing over others is the idea he was pushing back then, and if you read the ransom note it taunts the Ramseys and esp. John Ramsey, and makes all kinds of braggadocios statements that are totally beside the point of a mere ransom plot...or even a mere sex crime, for that matter.
SCTC means nothing. It's not in the hostage letter either. And "Shall Be The Conqueror" is really a stretch in my book.