Some say the mysterious predator with an appetite for great white sharks was the famed giant squid (Architeuthis). It isn’t the first time someone in that family has eaten a shark (see this), and giant squid are known to live in deep depths like those seen in the tag.
Sharks have returned the favor of eating squid, too.
However, it should be noted a squid's internal temperature is the temperature of the water it lives in. The beak is too small to ingest the tag, as well. Given that fact, it probably is not a squid.
Talking about facts, it is known that whales, like orcas (Orcinus orca), do prey on great white sharks, but do not dive down that deep (to my knowledge at least). Orcas generally only take the liver, as well. Sperm whales, though, do dive that deep in search of food (e.g. giant squid).
Others claim it was a cannibalistic great white shark that ate the “smaller” shark (although a 9 foot shark isn't exactly small).
This occurrence was the subject of the documentary, “The Search for the Ocean’s Super Predator.”
In this documentary, they also narrowed the suspects down to “squid,” “orca” and “great white shark.” From what I saw, the main suspect was the shark.
What do you think it was?
(Please don't say Carcharocles megalodon. You can say anything else but Megalodon.)