Maybe he'd given too much away. Perhaps his arrogance, which had permeated every one of his lives, had betrayed his secret. If they found out, it could mean the deaths of millions of Carcerinians; yes some were still alive. If he remembered his history correctly, then some time ago, the population had moved deep underground to escape a century-long corona flare, and many chose to remain underground, having formed a self sufficient near-utopian society that lived there for generations.
A quick scan from the TARDIS computer confirmed the existence of several million life forms below; the remains of the Carcerin population. The links between the surface and the world below had been severed a while ago, but a few abandoned vertical bridges still remained, although closed. For now, the daleks and the sontarans had no idea what stayed sheltered below, but if they understood what he'd let slip, everyone underground could soon be extinct.
Although if this plan worked, he could be responsible for the extinction of two more species; the entire Dalek and Sontaran species had gathered from across the stars to exterminate the Doctor. Every single wretched specimen was here; ready to fall into his trap. With a single sequence of numbers and switches, every single one of them would be wiped from the face of the universe.
But do I have the right? He asked himself. However vile they may be, did they have the right to life as a species? On the other hand, could he allow them to continue; they'd go on to destroy every life form they could. Even now, they were probably drilling to find any survivors. But so much more blood would be on his hands. He'd already attempted genocide on the Daleks on multiple occasions, but the vermin had always come back like a particularly murderous hateful rash, but to erase them forever with no chance of remorse? Could he commit two genocides for the sake of preventing one?
Actually, it was worse than that; if he put his plan into motion, there would be more than two species at risk; his own life was in the balance. The Time Lords could die out a second time, once again by his own hand. He'd been willing to risk his life before in thousands of situations; in a few of them, others had sacrificed themselves for him. His time had come. The Time Lords' time had come. He'd prevented the Time Lords from wiping out other species and activating the ultimate sanction by ending the last great time war and sealing them in a time lock to burn for all eternity; but now it seemed he was doomed to repeat their fate. He'd never wanted to be like them. This was why he ran away; all the red tape, corruption, power no species had the right to use.
But despite everything, despite everything he hated about them, he always wanted to rebuild the Time Lord race, but guide them towards the light, show them the wonders of the universe rather than cooping them up on a single planet, in a glass dome, assuming their own superiority because they could see so little of the rest of the universe. If they could travel, they would understand the universe, learn from it, and be more like him. But he couldn't repopulate the race on his own. He was all alone, with no-one to understand his burden.
All his life, there was never anyone quite like him, apart from Susan's grandmother. She was a traveller like him, before him, even. He'd never even thought about leaving Gallifrey until he met her and if she hadn't disappeared, he'd never have stolen (borrowed) the TARDIS in the first place. He'd never found her. Even if he did, would she even recognise him now- would she still love him now he was a serial killer?
Maybe this was how he'd find out- find her. Travel to the one world he'd never been to. He could make the world a better place with this one final act. The universe would be better off without the Sontarans, without the Daleks, without the Doctor.
And he could do it right now. He was the Doctor, and he was curing the universe's disease, exterminating the Dalek plague- but it would require something he'd never managed before in all his years.
He'd have to fix the chameleon circuit.