A few No Mercy Runs happen in the Storyspin AU. Frisk, getting cocky, reminds Asgore of each time he's failed to stop them, and that every time he does kill them, they just return via the SAVE System anyways. (Keep in mind Asgore nodding during his battle when you tell him how many times he's killed you after you've died to him.)
Each Reset, Asgore's sense of Deja Vu grows stronger. So during one fateful run, a few days before Frisk is set to fall into Mt. Ebott, Asgore does a bit of reading into timelines, looking through both Napstablook's and a much-forgotten former Royal Scientist's notes. Asgore soon gets better at calculating Resets and when they happen, much like Sans was in the base game.
Even with Asgore's newfound knowledge, he still always fails. That's when Frisk's final taunt to a dying Asgore strikes the biggest nerve.
"The only way someone like you would EVER be able to defeat me, is to sink as low to my level."
During the next Reset, and after having exhausted every option possible in past runs, he comes to one conclusion; He really does have to stoop down to the level that his own enemy has.
He begins to slaughter his own kind to try and gain EXP and LV for himself.
He becomes more distant and quiet, wearing an outfit right for a funeral, as if grieving every kill he commits. The very last people he'd want to kill was his own family. But eventually, he's forced to, as Toriel blocks the way.
She chews Asgore out, calling him a failure and a pathetic whelp. Asgore finally speaks up about his actions, trying to explain to Toriel the situation, but she's not having it. So that's when Asgore is forced to kill her. He keeps composure, but dies on the inside.
His guilt-ridden imagination conjures up a version of Toriel, one that's sickeningly cruel to Asgore. It always mocks him, taunting him like the human has done so many times before, and berating him for every failure. Asgore still remains silent, thinking that this punishment is deserved for what he has done.