Barbara Gordon easily could have died doing what she did. So many things could have gone wrong, for in reality she didn't have much of a plan, and her technology was stolen from abandoned prototype equipment scavenged from LexCorp and Cadmus. She was entirely on her own, for she knew that Bruce wouldn't agree with her plan and would do everything in his power to stop her, including sending Terry against her if need be.
Both Batmen, young and old, noticed that she had been a bit off in the months following the return of the Joker, but thought nothing of it.
She was glad they didn't.
Watching Tim suffer horribly for the second time, through having a copy of the Joker completely take over his mind and body, had burned the last fuse in Barbara's mind. He had never completely recovered from the incident, but what stuck with him most throughout the years was that he had broken the Bat-Clan's most sacred oath: Do not kill. He never forgave himself for doing so, even if his victim had been the Joker.
So she came up with an idea: To send her mind back in time to her younger self, and prevent it from ever happening. Time-travel technology was highly illegal and what little of it was allowed was highly supervised by the Justice League Unlimited, so she had to turn to some very shady and corrupt characters to get what she needed. Being the Police Commissioner definitely had its perks: She could look the other way on certain matters. It wasn't like the world she lived in mattered anymore, since she planned to overwrite it with a new one.
A part of her mind screamed that she was wrong and to stop her madness, but then another reminded her of poor Tim Drake and silenced the protests. There was no going back now.
She was going to send herself back in time, to not long before Robin was abducted by Harley Quinn. She would make sure that she was there to stop it. This was the one event that shattered the Bat-family, Bruce in particular, and nothing had ever been the same after that. He quit the League and shoved everyone away from him, eventually growing into a bitter and lonely old man. Even Wonder Woman lost her patience after so many years and left Earth, brokenhearted and full of pity and sadness at what Bruce had become. Decades later, she still had never returned.
Everything had gone wrong that night. Tim's life was ruined. Bruce's life was ruined. Diana's life was ruined. Her life was ruined.
Sure, she may have in time married and had a normal career and life, but guilt constantly plagued her. She had to be heavily medicated in order to reduce her nightmares, which never ever stopped.
Over forty years of torment and taking the same pills over and over that really didn't do anything to help.
Barbara placed her jury-rigged helmet upon her head. There was no way to test the device to see if it even worked... chances were high that she would either end up dead or a drooling vegetable for the rest of her life. It was a risk that she had to take.
Here goes nothing...
PAIN DEAR LORD THE PAIN THE PAIN THE PAIN!
At first, she thought that the device had indeed malfunctioned. She reached up and tried to take it off her head, only to discover that it wasn't even there. Her head felt like it was on fire and ready to explode, or that Superman was using his heat-vision on her forehead. It was too much, and then...
It just stopped. Just like that.
In an instant, she found herself in the middle of a battle by being kicked in the face, knocking her to the ground. Instinct kicked in, and within seconds she was on her feet again. Her body felt much younger and stronger. It had worked!
Her enthusiasm fell when she realized where she was, and just who she was fighting.
No!
She was outside of the old Arkham Asylum, meaning that she had failed in her original plan to save Tim from being tortured. The machine hadn't worked properly, and she arrived several weeks too late. Tim had gone through it all, yet again.
But that wasn't the worst part... she was fighting Batgirl.
A brief glance downwards at a puddle confirmed her fears. A woman in a red and black jester's outfit stared back at her. Forgetting that Batgirl was still trying to subdue her, she sank to her knees and screamed, and then wept.
Batgirl knew that Harley was insane, but had never seen her act that way before. It could have been an act to catch her off her guard, but she didn't know for sure... it was as if she had suddenly changed from one moment to the next.
Barbara, meanwhile, was fighting to regain her composure and memories of that night, with calming techniques that Bruce had taught to her himself. Within minutes, the Joker would be dead by Tim's hand... she still had a chance to stop that! Her mind was already formulating a plan to do so, a simple but quite effective and terrible one.
There was still Batgirl to take care of, though. She likely wouldn't believe anything she had to say in her current form, but had to do something in order to get away from her. She knew herself better than anyone, after all.
Getting up from the ground, she said the one word that she knew would get the other woman's attention. "Barbara."
Batgirl was stunned momentarily, until she remembered that Tim had likely told everything to the Joker and Harley, including her civilian identity. She didn't try to hide the truth, too much was at stake. "What?"
Barbara decided to tell the complete truth, at least about what she was going to do within the next few minutes. Mimicking Harley's speech to the best of her ability, she said: "I'm gonna kill Mista J. I should've stopped him, but ya don't really know what he's like. What he did ta tha kid was too much, you're right. After this I'm done, ya hear me? No more fightin' or anythin'. I should've listened to Bats years ago and got away from him, but it's too late. Now I can't have ya comin' after me, so nighty-night Batgirl!"
Batgirl, having fought Harley numerous times in the past, knew all of her moves and tricks. What she didn't expect was for her to move so fast and use one of her own take-down moves against her, putting her down for the count. Her last thoughts before losing consciousness was just how she learned to do it, which was a mystery that wasn't solved until days later.
Barbara arrived just in time. Tim hadn't done the awful deed yet, but he was about ready to. Batman was subdued and she knew from Bruce's story of that night that very, very soon the Joker would hand him the gun that would change everyone's lives forever.
Not this time.
The Joker noticed her enter and welcomed her with a horrid smile, with one arm around his son. She tried her hardest to not lose her cool and blow her cover, and only barely managed to do so.
"Ah, the rest of our lovely family returns! Just in time, too!"
"You're right, Puddn'! I was just taking out some Bat-Trash before returning to our lovely abode!"
Barbara couldn't stand hearing Harley's voice saying those words, or her entire act. It would soon be over, she just had to get close enough to him and make her move. "Aw, does Brucie miss his widdle Batgirl? You won't be seein' her anymore, unless she can survive falling hundreds of feet!"
Both she and the Joker laughed at the same time, watching Batman break further and further.
"Well done! She was rather a bit of a pest, I should say. But onto more important things... J.J. here was about to do the honors of putting an end to Bats here. I'd do it myself, but this is infinitely more satisfying! Robin killing Batman, ehehehehehe! If you would, Harley..."
Instead of getting it himself, The Joker wanted Harley to retrieve the gun and hand it to Tim.
This is beyond perfect! Not much longer...
Indeed, what happened in the next ten seconds was something that nobody had ever expected or taken any time to even account of. Harley Quinn took the 'Flag-Gun', walked calmly over to the Joker, and shot him in the middle of the forehead. He was dead before realizing what had ever happened, lying on the floor with his eyes wide and a flag with the word Bang! sticking out of his forehead.
She dropped the gun, and silence reigned in the room for the course of a minute. Nobody spoke, and Tim didn't even laugh. It was enough to shock him back into temporary sanity, for the time being.
"Batgirl isn't dead, Batman." she spoke, with her normal tone of voice. No more acting like Harley. "That was just a lie so I could get closer. I knew what he was about to have Tim do, and I couldn't let him. I'm so sorry, so sorry for everything... I should have listened to you all these years ago! But not for one moment do I regret killing that bastard, more than you'll ever know!"
Barbara couldn't speak anymore, for she found that her own tears were preventing her. She broke down into a mess, over Tim and over Bruce and over the fact that she hadn't completely failed.
"Batman, when you get Robin out of here... he has some kind of chip implanted in his neck. You won't be able to find it by normal means, but keep searching. You have to destroy that, it's a backup plan in case he failed!"
She untied Batman and quietly and calmly sat on the floor. He asked one simple question, one word. "Why?"
"Why? Because I'm tired of living with regrets. I should have stopped him, Batman! Doing horrible things like that to a kid! Now he won't be able to do it to anyone ever again, including me. Hell, you, Batgirl and even Ivy kept trying to drill it into my thick skull that he truly doesn't care! Just... just look after Tim, okay? Get that chip out of him."
"You are not Harley. I don't know who you are, but you are not Harley. Who are you?"
"I guess you figured it out sooner than you should have, but I always underestimated you. Harley is dead. I'm actually Barbara Gordon from the future, stuck in her body, because I came back in time like an idiot to try and stop Tim from ever being tortured. It ruined all our lives, Bruce! And then that stupid chip kicked in and literally turned him into the Joker! He was lucky that Terry was able to get rid of it without killing him! And I didn't come back in time, Tim had to go through that all over again and..."
She didn't get to finish, as a recovered Batgirl had creeped in from behind without either her or Batman noticing, and knocked her out.
"That's for earlier." she said, but her expression quickly turned to horror at seeing the dead body of the Joker on the floor, and a nearly-catatonic Tim standing a few feet away. "My god... what happened here?"
"That's something I'm still trying to figure out." said Batman.
Barbara could no longer be Barbara, since there was another one in that time line. Besides, she no longer had her original body, and decided to just stick with the name of Harley. She would spend however many months or years or so in Arkham, and would then be free.
Harley was a hero to people on both sides of the law for finally cutting out the cancer that was The Joker. She claimed to finally come to her senses and killed him in self-defense, but nobody truly believed her. Despite this, she avoided prison time, with great help from Batman himself.
He questioned her for hours upon hours, and even resorted to getting the Martian Manhunter to read her mind. She was telling the truth about everything.
Above all else, she warned him to properly look after Tim's recovery, and his own. In the original time line, he had alienated everyone until he was left alone entirely, even more so after Alfred's death.
Harley encouraged him to go after Diana. She always knew that he loved her, and regretted denying it for the rest of his life. He didn't comment either good or bad, but she hoped beyond all hope that he'd consider it.
As for herself, she had a completely new life to look forward to once she was released from the Asylum. She didn't know who she was going to be or what she was going to do, but she hoped that she made a difference for Tim, Bruce, the other Barbara... everyone.
The nightmares ended, and the sun rose.