There really isn't one. It seems like the best motive "TH Did It" people use is from the 4-perp theory that the victims were spying on TH, DJ, LGH and BL doing drugs (and/or engaging in sexual activity) in the Robin Hood Hills -- and they killed the boys when they discovered they were spying on them.
This "theory" was produced by a neighbor of BL's who was a drug dealer and had a long history with crime (real reliable source, lol). He more than likely was simply trying to provide police "information" (however bogus) in case he ever got busted again.
The theory is utterly ludicrous because, it states that TH/DJ picked up LGH/BL -- even though the two groups didn't know each other and had never met before this day. So not only did they decide to pick up two unknown teenagers, but they also decided to commit a triple-child-homicide with them -- even though they had never met before.
And also, any drug dealer would have freaked out if one of his regular clients (in this case, BL) dropped his full name to two unknown adults who were supposedly looking for weed, because they very easily could have been undercover. This isn't even the worst hole in this theory -- there are so many. It's beyond bogus, but there are people out there who believe it as fact.
https://famous-trials.com/legacyftrials/memphis3/wm3-pam-hobbs-and-john-mark-byers-court-filing.pdf