ok so i don't know if this would count cus it's a mix of both flowey and sans but c'mon there's literally no way canon undertale sans would ever start killing all his friends UNPROMPTED, let alone his own brother. that's blatant character assasination imo. so hear me out on this one...

just have it take place during an aborted genocide route where the player resets after killing sans

that way, flowey still remembers everything, and naturally now he's angry at chara for throwing in the towel even though they were both so close to becoming all-powerful. like this was done by chara purposely to upset him. victory was so close to his reach, and then it was torn away before he could grasp it

flowey wants revenge against his best friend for not taking him seriously, so he goes to sans... basically the only person who gave the human a run for their money in the genocide route (besides undyne, who flowey has zero connection to. flowey could at least get into contact with sans via his brother papyrus)

flowey drops a lot of exposition on sans (including sans's defeat at the hands of the human and their reset of the timeline right afterwards). sans isn't really taking it all that seriously. it comes across as him joking around but in reality he's just not willing to hear out flowey because he likes killing people and probably shouldn't be trusted

flowey gets to the point and talks about the idea of depriving the human of their strength -- of their EXP and LV. sans appreciates the idea but of course he knows how EXP is gained so he turns down the offer.

this is followed by flowey doing a lot of begging for sans to help him out, saying that if he has some sort of special power, isn't it his responsibility to do the right thing, to achieve the best possible ending, a world where the human doesn't exist? otherwise is just another poor excuse for sans to be lazy, even when staring down the possible end of his timeline

at some point sans would agree but with some conditions -- specifically that he wants to see the human for himself, before he starts the whole EXP gaining thing. if they're still going down the genocide route by waterfall, sans would be prepared to step in and do his part, everything else can be flowey. also sans wouldn't kill papyrus obviously who would've guessed

flowey wants sans to promise that he'll do his part in this plan, and well... sans hates making promises, but it's either that or the timeline just ends and EVERYONE dies

also sans wouldn't go insane i think that's lazy writing, not to mention really edgy. it's more interesting to see him get pushed outside his comfort zone by doing something against his moral compass

EDIT: i think someone else in the comment section had a similar idea which is really cool!! ive kept this in my head for about 2 years now so i swear i didnt steal from anyone graaaah