The following article contains spoilers from Future State: Gotham #13, on sale now.
Damian Wayne's fate in the future has finally been revealed in Future State: Gotham #13 (by Dennis Culver, Geoffo, Troy Peteri). His parents discover Damian descended into Hell to try and find Batman. Unfortunately, his time there accelerated his aging and corrupted him. He now seeks vengeance on all of Gotham for turning its back on his family.
His situation, in some ways, mirrors what became of his best friend, Jon Kent. Both were trapped in hellscapes and aged at a faster rate. Unlike Jon, though, Damian could not hold onto his humanity, becoming perhaps Gotham's most dangerous villain. The similar experiences, but different outcomes, suggest that their upbringing determined how they returned. In this case, Superman's parenting style seems to be better than Batman's.
A few years back, while on a trip with his grandfather, Jor-El, Jon became trapped on Earth-3. Ultraman had trapped him in a volcanic prison to use Jon as a form of therapy. In Jon, Ultraman saw something of an equal, another Kryptonian who might be able to comprehend how distanced Ultraman felt from the rest of the world. He became Jon's only living contact and never a pleasant one until eventually, Jon mustered the strength to break free of his prison.
Throughout, the only thing keeping Jon from losing himself was the hope and strength his parents had managed to teach him before they parted ways. The thought of getting home or that they were looking for him kept him sane. Despite the trauma of the ordeal, he still retained his inherent goodness and has become the Superman that people love today.
Damian's situation was eerily similar. He went into Hell, knowing that there was probably no way back. While it is unknown what he faced down there, it isn't called Hell because it is pleasant. Damian most likely experienced untold horrors in his attempts to find his father, all of which would have been in vain, until finally, it all broke him.
Batman never really bolstered Damian's humanity the way Jon's parents did. Granted, he did help Damian discover his morality and compassion, but he could only ever pass down his cynicism about the world to Damian. Unfortunately, that lack of faith in people or their ability to improve is not the defense he needed. After all, everyone in Hell is already truly evil. There would be no hope or goodness to find there.
In Batman's defense, there are some critical differences between what happened to Jon and Damian. During Jon's imprisonment, Ultraman never actively tried to torture him, just talked to him in his own broken and toxic way. In addition, Jon held onto the hope that someone would come for him. Damian didn't have either of these factors helping him. Every demon in Hell would have actively tried to torment him. Damian didn't go in expecting to rescue because he assumed he was the rescuer. Then he learned there was no one to save and that he had trapped himself for nothing.
Ultimately, it comes down to their respective abilities to hold onto hope. Jon's parents practically live on their hope for the future, so it's no surprise that Jon wouldn't give it up for anything. Damian, however, has only rarely been shown the good in people. Going to a place that reinforces this belief would have slowly whittled away at him until he became the monster he was always afraid he'd become. If Batman had not failed him, this wouldn't be happening right now.