The Morgan theory makes no sense. How would he know about the zombie pit? He finds the pit, how does he know it belongs to Woodbury? I feel like they pretty much suggested it was Milton at the end of last episode... Morgan doesn't make sense and would be hard to logically write that into the story. But it ant like these writers are on point much anyways so some weird left field shyt like that might happen.
I still think it is Milton though. His skittish demeanor is the writer's attempt to throw us off so when he is apart of some gully shyt, it will be a suprise.
i don't believe in the morgan theory but it is plausible. rick told morgan where to find him if he ever wanted join them. morgan could have come across the zombie pit and set them on fire. it doesn't matter if they belonged to woodbury, morgan currently feels it's his duty to clear(burn) all zombies.
now if it were morgan, he would have found the prison by now since it's been days since the zombie pit burned. also morgan is listed on imdb as being in the season finale. although according to the actor who plays him, he has no idea why he's up there.