WARNING: Spoilers Ahead For The Walking Dead Midseason Finale!
It's clear that Dwight and Negan's torture did a number on Daryl, as evidenced by how fully insane he went when Fat Joey got in his way.
While Daryl was escaping, Fat Joey happened to catch him outside. Instead of trying to stop him, Joey put up his hands and let Daryl walk away a free man. That wasn't good enough, and Daryl beat Fat Joey's head in with a pipe.
Some fans probably cheered at this brutal kill, mainly because it meant Daryl was far from Negan's pet, but also because it was one less Savior in the world.
It makes sense to be excited for this, but Fat Joey's death isn't a good thing - not even a little bit.
What many don't realize - mainly because the dialogue was somewhat hidden - is that Fat Joey was actually incredibly smart. In fact, Joey was one of the only people keeping all of the communities safe, and his death means bad news for Rick and the others.
Two episodes ago, when Carl and Jesus were hiding in the truck, the two Saviors who were commandeering the vehicle were watching some walkers cross the road in the distance. If you were listening closely, one of the men explained that there was a herd nearby, and they were being led away from the communities by a series of explosives.
The man said that Fat Joey was the mind behind the 'redirect', and that he was using a trail of trip-wires and RPGs to alter the course of the herd, and keep them away from everyone.
When the truck arrived at Sanctuary, and Carl was readying his gun, Negan could be heard talking to Fat Joey outside. Joey told Negan that something had gone wrong with the redirect, and that he was working on fixing it.
Negan later goes in to talk with Sherry about Mark and Amber. When he confronts her, he states that Mark skipped out on redirect duty to be with his former lover. This is where the mistake come from.
So, in short, Fat Joey was in charge of keeping everyone safe from a nearby herd of walkers; possibly bigger than any they've faced before. Now that Daryl's killed him, trouble could be brewing for Rick and the others.