Dr. Demented didn't create the Time Key. The Time Key was created picoseconds after the creation of the universe, by beings who lived before electroweak symmetry breaking. The cooling universe destroyed every last remnant of their world, save one highly advanced machine. There are no words for most of the machine's components, but its heart was what we would call a black hole, its brain was what we would call a quantum computer, and its immune system was what we would call a doomsday device.
Similarly, there are no words for the Time Key's intended purpose. It was a very important component of a machine (or was it an individual? Or a civilization?) never completed.
Dr. Demented hadn't created the Time Key, but he had refined it. He had long since forgotten where he discovered it, but he still remembered his first experiments using it. Learning the machine's potential to bend space, and create doorways. It could be used to open gates from one end of the universe to the other, or even to reach the very end of thermodynamic time.
What the Time Key could not do, at least not initially, was open a doorway back in time. This is because time paradoxes are impossible, and travelling into the past nearly guarantees a paradox. Hence, traveling into the past is nearly impossible.
Dr. Demented had seized upon the word 'nearly.' His armor could attempt time jumps at a truly prodigious rate. After a few quadrillions of quadrillions of quadrillions of quadrillions of quadrillions of attempts, which would take the Time Key several years, it was sometimes possible to make a jump.
You couldn't actually change the past, of course. That would create a paradox. A jump further in time would require more time to prepare. Carrying more information would also be more difficult. As would carrying more mass, more power, or more distortions in space-time. All of this meant that it was very difficult for Dr. Demented to go back in time.
He had done it in several occasions. He had witnessed the Time Key's creation, at a time when a man of Dr. Demented's size would have taken up much of the universe. He had traveled to see the heat death of the cosmos, and had returned to complain about it. He had walked with the last dinosaurs, with the first Computer People, and the humans at the peak of their civilization. He had led bands of hunters, and conquered galaxies. But of all the people he had met in all those times and places, he only truly cared for one.
No, it wasn't himself. Dr. Demented had evolved past the need for self-preservation. Automatic programs, more complex than the human mind and more ancient than the stars, kept his body and mind in working order, protecting him from those who would do him harm.
No, the one person Dr. Demented cared about was Nimue. One of the Computer People. Part of a race of superintelligent Space Gods. The Doctor had met her while raiding their Technopolis, a university for gods, housed in the core of a neutron star. It had worked out surprisingly well for him. Not only had he stolen the fire from the almost literal gods, he had also absconded with a lifelong companion. He had used his newfound knowledge, combined with other scientific lore gleaned from experiments across time and space, to empower Nimue, giving her abilities to rival his own. Incidentally, this goes against the Mad Science safety rules: never give out superpowers without giving out a self-destruct mechanism.
Unfortunately for everyone in the universe, those powers did turn out to be the death of Nimue. The very nanomachines that let her manipulate water turned her blood to poison (metaphorically speaking, of course. Her body didn't have actual blood). The poison, like the machines that created it, were inextricably linked with her body and brain. There was no way to remove it.
It was impossible to change the past. Even the Time Key couldn't prevent things that had already happened. But Dr. Demented's mind was already far afield. In his madness, he created an unstable pocket dimension. One where Nimue was still alive, and he ruled the Earth.
Even that didn't work out. Nimue realized the brilliant beast ruling the planet was no longer the man she loved. She did her best to destroy him, and the threat he represented to both Earths. Eventually the second Earth collapsed and, thanks to most of Earth-Alpha's heroes and villains, Dr. Demented was caught in the dying timeline.
Too bad he could survive the destruction of a mere universe. Dr. Demented was back, and he was going to undo the meddling that had ruined his existence. He was going to set things right, and and nothing could separate him from Nimue.
The Time Key wasn't like the Crucible. The Crucible was quasi-sentient machine, slowly paving the way for its host to conquer the universe. The Time Key was also somewhat sentient. But its creators had lived out their entire lives over the course of zeptoseconds. The Key was programmed to think on the same time scale. The machine had lasted untold billions of times longer than it should have. Even as it sailed through the age of the universe, the Time Key drowned in a sea of eternity every single second.
It still remembered its original purpose, and knew that that purpose had been fulfilled. Now, the Time Key was mutated into something new, something strange. It had been torn apart, combined with a hundred pieces of arcane technology that only Dr. Demented could understand. It lived in an ethereal world, empty by the standards of its home in the early universe. And, if a machine like the Time Key can want anything at all, it wanted its overlong existence to cease.