He honestly didn't know what had possessed him to do it. Perhaps it was because the Brigadier hadn't stopped with the prying questions about him and his species after that whole incident with Omega in which his two prior selves had gotten involved. Either way it had been exceedingly childish of him, even if seeing the Brigadier just about have a heart attack had been rather amusing.
The Brigadier had asked him any number of questions about Gallifrey and its people in the beginning, but had soon shown how good a friend he was and stopped after he'd shut him down with an "I've been exiled" every time he'd started asking anything he didn't care to answer. This time, since his exile had been lifted, he did not have that excuse to fall back on.
"And, how does your species, ah, make more of your kind?" the Brigadier had asked an hour earlier, providing him with the perfect opening.
"We reproduce asexually." he'd said, as he found his mind harkening back to his school days and the mischief he used to get into and starting to plot out temporal coordinates.
What he'd said to the Brigadier had been true enough, what with all of the families who chose to use the Looms rather than go through the ordeal of illness, swelling, stretch-marks, and the tending to a baby and later small child, which was traditionally the purview of the male. The Looms which popped out full-grown individuals had taken a great deal of stress off of a large number of fathers over the past ten million years since they'd started coming into use.
What he'd said to the Brigadier ten minutes ago hadn't been true however.
Creating a time loop that could potentially become unstable due to the unpredictability of Human beings was an awful risk, especially for a prank, but he'd found that he couldn't resist.
The line "We reproduce asexually." had been his cue to step out of the shadows and say "The process is rather fascinating Brigadier. One morning you just wake up and find that there's two of you."
If Lethbridge-Stewart had had a gun in his possession right then, there was a good possibility that one or both of himself would've been shot, which would've been exceedingly disastrous if it had been his past self seeing as there needed to be a him to go back in time in order to step out of the shadows and scare the Brigadier. Fortunately for the universe, the Brigadier hadn't had a gun, and therefore had a panic attack instead.
As he was getting the poor man to start breathing again before he passed out, he gave his past self an opportunity to depart.
He was pretty sure that what he planed on doing if the Brigadier didn't twig onto the trick he'd just pulled on him and started asking more questions would make the man stop asking about him and his kind altogether.
The next day:
"Doctor, where is the other fellow?" Brigadier Lethbridge-Stewart asked when he came into the lab after he'd finally started to wrap his head around the other Doctor who he would need to start filing paperwork on and only saw one of the Doctor.
"I ate him." the Doctor replied, not looking up from his experiment.
"You what?!" he exclaimed.
"My dear Brigadier, why do you find the fact that I ate him so shocking?" The Doctor asked, finally looking up from whatever the hell he was fiddling with.
"You ate a...He was...You ate..." he said, completely at a loss for words.
"I seem to have forgotten that your kind don't eat their parents. It's such an integral part of our life cycle that we don't even bother to think about it." the Doctor said, going back to his experiment as if nothing had happened.
About the only thing that carried him out of the lab without braining the creature in front of him after that was that he'd only just barely reminded himself that despite all outward appearances, the Doctor, the late Doctor had been an alien and his ways weren't the same as theirs.
He'd been at his desk and hour later looking down at the paperwork he'd needed to do regarding the Doctor's death and trying to figure out what to do with the Doctor's cannibal offspring when Captain Yates came into the room with a reel of tape.
"What is it?" he yelled, his grief at the Doctor's passing turning to anger.
"Sir, I think you might want to listen to this..." Yates replied.
"What is it?" he asked, not particularly wanting to listen, but noticing that Yates seemed to think it was very important he do so.
"You remember when you had us bug the Doctor's phone shortly after he arrived?" Yates asked.
"Yes," he replied, remembering how he hadn't entirely trusted the Doctor in the beginning and realizing that he apparently hadn't rescinded that order considering the tape that Yates was carrying.
Yates loaded the tape onto the bulky tape player and started it up. It was the fellow in the lab calling the Master who'd sounded surprised as hell to hear from him, especially in so friendly a manner. But, then again, being of the same species, the Master was the only other person that the person on the other end could have such a conversation with. A conversation that he and Yates were eavesdropping on.
"...And then, I told him I ate him." the Doctor who was indeed alive said, causing the Master to laugh.
"He will pay for this." he said, his grief turning to anger once more, but for an entirely different reason this time, when the conversation ended and the tape clicked to a stop.
A week later Brigadier Alistair Gordon Lethbridge-Stewart could categorically state that starting a prank war with the owner of a time machine was a supremely bad idea.