The following article contains spoilers from Future State: Gotham #16, on sale now.
Ever since Damian Wayne returned from Hell, something has been off about him. It seemed safe to assume that spending so much time there had altered his personality or even caused something in his mind to break, but Future State: Gotham #16 (by Dennis Culver, Geoffo, and ALW's Troy Peteri) hints at something darker. There exists a chance that Damian returned from Hell carrying an unknown passenger within him.
His actions, while certainly understandable (to a degree) don't seem entirely of his own volition. Hush's attempts to manipulate him aside, the issue had moments where Damian expressed doubts about a certain course of action before a sadistic glee seemed to overcome him, completely overriding any misgivings he may have. It certainly seems to support that Batman 666 is demonically possessed.
Currently, not much is known about the details of Damian's time in hell. All that's certain is that every day was a struggle for survival, and he eventually found out his torment was for nothing. After that, not much is certain, especially how he managed to escape Hell. Considering where he was coming from, and just how eager the demons there were to hurt him, it seems likely that the only way Damian could have escaped was to make a deal. Arrangements such as those are rarely straightforward and Damian might have been possessed without knowing it. The evidence for this seems to be in his own actions and words.
When Hush, while masquerading as Bruce Wayne, told Damian to kill an incapacitated Jace Fox, Damian hesitated, wondering if this was the right thing to do. Then, all of a sudden, he was smiling, a cruel excitement replacing the doubt. His next words though, "Finally, Batman dies by my hand." are the strongest evidence of possession. Damian went into Hell with the express purpose of finding his father, not killing him. The glee here, and use of the word "finally" indicates that this was a long-awaited outcome, but it couldn't possibly be Damian's. So, the question now stands: Who is possessing Batman's son?
The simple truth: there are too many possibilities. From Barbatos to Neron himself, a good many demons would have reason to want Batman dead. What is certain is that this is some kind of grudge from Hell itself. Damian is just an unfortunate means of bringing it to an end. While this may be a problem for him, there is a silver lining.
If Damian is possessed, then that means his actions and the rage he feels towards everything, are not entirely his own. The son of Batman's true self is fighting against these dark impulses, but is being suppressed by both the demon possessing him, and the manipulations of Hush. The fact that he is fighting it though supports a happy ending for him where he is free of Hell's control and reunited with his family. Nightwing's vision of the future may come to pass after all, but it all depends on excising whatever has taken hold of Damian's soul.