I also think one have to take into account the various aspects of the situation they are in. Daryl clearly shows that he has lost his footing when roaming the woods with Beth. He is angry, irritated, bitter and even more gloom then before. Almost like a feral animal. On top of this he is traveling around with a girl that is an emotional wreck and show need for reckless rebellious escapades. Pushing his buttons by trying to get some response.
He also show his level of frustration and anger when, instead of terminating one of the walkers at the country club, beats it senseless first. Then later uses another as target practice. It all accumulates in the breakdown and short glimpse into his thoughts. First by the angry shouts of them all being dead. They are never going to see any of them again, and then his guilt comes to light by;
The Governor rolled right up to our gates. Maybe if I wouldn't have stopped looking. Maybe 'cause I gave up. That's on me.
And your dad. Maybe – maybe I could have done something.
He has also lost a place where he, perhaps for the first time in his life, had a role where people looked up to him. At the start of season four he is at a very good place. From there until the final stab with the fall of the prison it is nothing but a long beating tearing everything down.
The episode also starts out with them both taking refuge in the trunk of a car. Intensely on watch throughout the night with sights on the aim of the crossbow. Inside the clubhouse one also see that the people in there has "opted out". (Or it can also have been some lynch mob going bananas?) Throughout the series we clearly get that that is something he really despise.
In many aspects he sees himself back to scratch. Back to were he was before it all started. Drifting around. Nobody. Nothing. In his own words; a red-neck a*hole.
Now. With that as the stage I do not find it strange he behaves erratic and out of place, even so long after it all started. In many ways it is revenge on the situation he finds himself in. It is a melt down. A blow out.