Karl's obsession with taking down Billy Mitchell was retarded to begin with. What is the benefit aside from money and ego stroking?
Karl made a lot of money destroying one of Billy's friends from back in the 80's, Todd Rogers.
Todd was a Twin Galaxies referee for a long time and was present for a lot of Billy's achievements. He held hundreds of retro arcade records for decades and was generally considered to be the arcade GOAT throughout the 80s, 90s, and early 00's. The big problem was Todd was a gigantic cheater and liar, and basically all his records from the 80s were fabricated. He used his insider position at Twin Galaxies to force shit through without proper validation and it came back to bite him in the ass in the last 2 decades. A younger generation with better technology and more autism poured through his old records and proved many were impossible.
The fallout was pretty huge in the retro community. It caused widespread skepticism about old and new records alike and spawned a group of hyper nerds that pour through useless vidya footage to prove other losers lack honor. It's how I found Karl's content. I believe Todd sued Karl at one point over Karl's multiple exposes talking about Todd's cheating. I can't remember if Karl won or Todd, who was already very poor, ran out of money. Either way, Karl totally annihilated Todd and totally destroyed any ability he had to earn a living off his past. He's probably sleeping on a pissy mattress in a trailer somewhere, but NOBODY is happier about Billy's win than Todd fucking Rogers.
At least to me, Karl made it seem like the lawsuit with Billy was going to be a 1:1 recreation of the lawsuit with Todd. In reality, Billy had financial security and could afford to see a lawsuit to judgement. Todd didn't. Also, Todd sued Karl over cheating accusations, and there was overwhelming proof that Todd was a cheater. Karl made it seem like Billy was suing him over cheating claims that also had overwhelming evidence (finger girders), when in reality, he was suing over the ApolloLegend suicide.
I think this was all by design. Karl got rich off ruining the few remaining years of a shitty old man that normies didn't know or care about. He turned him into a household name for millions of people, then drove his dick into the dirt. He thought Billy's lawsuit was Pay Day: Round 2. It looks more like karma has come calling after dragging Todd so hard. I'm not defending Todd. He is a piece of shit, but I never got rich of making him more famous than he could on his own.
TL;DR- Karl did it for money.