Notes: This is an idea by spookykingdomstarlight, thank you for letting me write it (badly)
I know BB-8 is male-programmed in the novelization, but I'm terribly attached to BB-8 going with "it."
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Droid Wars
It all started one day soon after BB-8 and Poe had joined the Resistance, during a routine briefing which included all higher-ranking officers currently on the D'Qar base.
"Say," Poe had asked in all his innocence after Andor from Intelligence had reported the non-classified parts of his latest mission with his droid partner K-2SO, "why isn't Andor in Droid Intelligence?"
BB-8, in its defense, tried to prevent the worst. It beeped shrilly and bumped with all its strength against Poe's leg to shush him.
"He's great with dro… ow! Beebee, stop it, why are you telling me to shush? I'm sure the General doesn't think it's disrespectful."
It had tried, but it was too little, too late.
Poor clueless Poe beamed when General Organa praised him for the suggestion, and C-3PO chimed in that while organic, Andor would be the perfect addition for his department with his reprogramming skills – reprogramming skills?! BB-8 rolled back in alarm and chirped shrilly that It Would Not Be Reprogrammed and Don't Let The Bad Man Touch It Poe!
Poe's hand came down on BB-8's head dome and caressed away his fear with a gentle touch and that soft, soothing voice half the droids on base loved already cooing, "He reprogrammed Imperial droids, Beebee, the ones that weren't free to think and feel as they could have. You don't have to be afraid. I'd never let anyone hurt you."
It was no surprise that every droid who met Poe Dameron adored him.
Every droid except for one, BB-8 noted now from its safe-from-Andor's-reprogramming spot under Poe's chair. Its optic zoomed in on K-2SO, who was glaring daggers at Poe with nothing short of sheer malice.
Two days later, BB-8 heard through the base's very lively droid grapevine that Cassian Andor of the Rogue One veteran fame had been reassigned to C-3PO and his network of droid spies. It was generally considered a well-suited assignment for the aging spy, as it would permit him to spend more time recruiting, planning and programming than risking his life on suicide missions better left to younger men with faster legs.
BB-8 knew better.
BB-8 knew that its Poe was doomed.
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If you asked BB-8, Poe was the best organic friend a droid could have.
If you asked C-3PO, or pretty much any other droid on base, they would agree that BB-8 had the Best Organic.
Poe could give the very best pettings from anyone on base and he crouched down to talk to astromechs on optic level. He always said please and thank you, and while he gave orders to droids when necessary, he only did so as a commander would to a soldier, not as a human would to a droid.
K-2SO's Andor, on the other hand, was a very irritating human. Call BB-8 biased against him for having engaged in reprogramming during his Rebel days, but Andor was curt, rude and never tried to pet BB-8. Not that it would let Andor anywhere near it, but this was a matter of pride and principle.
Andor was, however, K-2SO's human. And Poe had unwittingly been the cause of K-2SO having to share his human with the Resistance's entire network of droid spies.
Unless specifically programmed for it, droids weren't naturally hostile, but they were possessive of the few things they had and valued.
BB-8 wouldn't deny that Poe had it coming for him.
But Poe was BB-8's human, and BB-8 was just as protective as K-2SO.
It didn't act when Poe was unable to get a single hot shower anywhere on base while they worked fine for everyone else.
It didn't act when Black One was repainted in First Order colors.
It didn't even act when another faux poster showed up in the Resistance network, sent from an untraceable source and defacing Poe's beautiful hair with the First Order General's ugly ginger mop.
That was not to speak of all the little snubs its poor Poe suffered at the fiend's hand.
Poe, determined to make friends with every droid on base, tried time and again to engage with K-2SO in his normal friendly banter.
"The likelihood of catastrophic failure is 97%, Commander. I look forward to hearing your obituary," when Poe jokingly asked him to wish him good luck before a mission.
"Your security clearance is insufficient," when he asked after his day.
"You are not authorized for this sector, I'm obligated to warn you only once before I apply lethal force," when they had gone to deliver a flexi in person.
With every snub Poe grew sadder, but since he was Poe, he also grew more determined to win K-2SO over.
Once again, BB-8 tried to prevent the worst.
"Stay away from him, Poe, he doesn't like you," it even chirped at him in its exasperation. You couldn't get any blunter than that, but did Poe listen?
"I know some droids are wary of humans, but I'll just have to work harder to prove I'm not bad," Poe said with that fire in his eyes that made humans and droids alike swoon.
BB-8 didn't zap him.
Later, it would wish it had zapped him. A timely zapping, and the Great Droid Wars of D'Qar may have been prevented.
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There came a point when a droid had to take a stand.
For BB-8, this point came on the day when Poe returned from a solo mission fuming over a bug in his targeting computer, which didn't resolve itself until the very last moment. He had never been in actual danger of it failing, but had suffered the scare of a lifetime.
When K-2SO tried to use his recharge dock that night, he got a very painful electrical charge.
It took him, Cassian and a small squad of engineers two days to undo the damage.
That was part one of its revenge.
Part two involved some scheming worthy of an intelligence droid, and a lot of looking cute and innocent.
All things considered it hadn't been that hard to plant the seed which would lead to General Organa offering the spare recharge dock in her quarters; C-3PO didn't like to shut down for recharge and would be happy to have someone to talk to.
K-2SO couldn't pale, but BB-8 was certain it henceforth saw a flicker of panic in his optics whenever he caught sight of C-3PO.
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Three days later, BB-8's oil bath was spiked with heavy-duty glue.
Eight days later, K-2SO's routine firmware update reduced him to making chicken noises.
D'Qar's droid community started a betting pool.
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A full month and seven pranks later, a very alarmed Poe Dameron dragged Cassian Andor into the hangar and hissed at him, "this has got to stop before BB-8 gets hurt!"
BB-8, who was most certainly not eavesdropping but just so happened to be hiding in a place neither man could see from where they stood because droids happened to be in such places all the time, had to stifle his outraged beeps of betrayal. If anybody was going to get hurt it would be that giant brutish slag-processor!
"Your droid's been harassing mine for no reason, Dameron," Andor responded.
BB-8 rolled back and forth in its not-a-hiding-place.
"Whatever it is, Beebee didn't start it. You'll find no sweeter-natured astromech," Poe vowed.
BB-8 felt only slightly guilty when it thought of the fact that K-2SO was currently locked into the base's deep-freezing unit.
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It should have been the end of their feud, though Andor had convinced Poe to let the droids sort it out amongst themselves. Whatever their prank war was about, it would sort itself out soon enough, he'd promised.
He was right, for a while.
BB-8 was eventually sent to Jakku with Poe, and ran into Rey and Finn, and for a while his prank war was indeed forgotten.
Then Cassian Andor was sent into First Order territory to make contact with possible droid spy recruits and install the firewalls they'd need to hide their betrayal from routine scans. K-2SO had to stay behind.
The next morning BB-8 came out of recharge to a lovely new paint job in lime green and puce.
It raced through the base, swearing vengeance in shrillest tones, right until it bumped into R2-D2.
The older astromech wobbled in amusement at BB-8's tale of woe. He had been very upset that he missed the prank war.
"So. Are we going to kick that kriffing slagheap in the bolts or what?"
BB-8 chirped and lit its blow torch.
It. Was. On.
The End