Impossible

I was tired. I had just sent an entire interstellar empire packing. Sorry. I had just been tortured, killed, resurrected, and then, after decoding billions of alien documents, I had sent an entire interstellar empire packing. So I had a right to be tired.
The right, but not the opportunity. I was no longer being bombarded by strange lights from the outer solar system. But I did see hints of gravitational lensing upon distant starts. That meant that Dr. Demented had won.
What could I do? Could anyone help? The Computer People? Well, they could only help that the speed of light, and there was no reason to think any were nearby. Even the ones on their way to kick the Fortarians' asses were years away.
Earth? Could anyone on that little blue ball still help me? Help themselves against the threat in the outer solar system? They had defeated him once before. No. Last time, the Doctor had been defeated only by his own insanity.
Could I wait? Let him destroy the world? Impersonate Vafnir, gain he madman's trust? Then steal the Time Key and reverse the damage he had done? That seemed like a long shot.
Could I escape? Take this palace of alien knowledge and outlast Dr. Demented, then pry the Time Key from his hands as his mind turned to porridge. Places a lot of confidence in my ability to jump to the right point in time. Plus, I reminded myself, the past cannot be changed. So if the cavalry was about to come in the form of future me, present me could stick around safely hiding behind my godlike future self.
But time jumps are hard. I couldn't expect this to be one of the few moments which allowed intervention from the future. It needed intervention from me. How could I intervene? What could I do against Dr. Demented.
Nuclear bombs? No. Black Hole? No. Neutron star? No. Strangelet? No. Cosmic String? No. Proton shift? No. QCD beam? No.
"Don't you know," Lucy asked.
"You'll have to be more specific."
"Do you recognize him?"
"Dr. Demented? I was in the Timeless War."
Lucy sighed. "Dr. Demented is you."
It took me a second to process the information. A full second.

Was that even remotely possible? Could Dr. Demented be my future self? Well, what traits did Dr. Demented have? He was smart. Smarter than me. But it was perfectly reasonable that with further upgrades I could reach his stratospheric transhuman level of knowledge some time in the next few billions of years.
He was demented. Mad. Perhaps one of my modifications went wrong? Was that consistent with my personality? Yes. Taking risks in the pursuit of knowledge was my thing. Possibly my defining characteristic. It was a chilling thought that one day, in the oh-so-distant future, One of those risks would go so catastrophically wrong.
I went through my core programming. The parts of my mind I had made edit-free. Was the Doctor's behavior consistent with that? Self preservation? Check. Curiosity? Check. Other characteristics with no analogue in human thought? Check check check.
Did he look like me? Not especially. But appearance was trivial. Over the amount of time he had likely lived... he had probably worn a thousand faces from a hundred species.
Linguistic patterns. Consistent with Russian and English being my first two languages. Reasonable given what I knew of cyborg psychology. But I couldn't speak confidently on the matter. It occurred to me that by cyborg standards I was an infant. Just beginning my immortal life.
And now I was asking myself to end it? Commit suicide. Kill my future self in order to safeguard a planet full of humans.
What value were humans compared to myself? I could make new humans if I wanted to. I had the codes. Simple variations of the same genome. I couldn't kill myself to save them. I could not fight Dr. Demented.
"Sorry," Dr. Demented said. "But that is problem."