Was perhaps from the moment that Monsterkind were hurled down Mt Ebott like trash that W.D. Gaster knew humans were rotten creatures? Or maybe it was instead during the war when he had been but a young child. Forced to flee from all he's ever known as battle raged across lands, then later as he grew up to create weapons of mass destruction. Perhaps, it was before he was even born that he had know this truth. Gaster was, after all, one of the smartest monsters to have ever lived, and most certainly was the smartest monster in the Underground.
Because humans were dangerous, after all. They were miserable, miserable creatures that were powerful yes, but miserable.
And his next invention would be the bridge to that gap between their two races. No. Rather then gap it was a chasm.
He would create the first biological weapon, and once the barrier was destroyed then nothing would get in its way.
Reader insert. What if W.D Gaster didn't just get split between time and space, but actually ended up in your world in an abandoned hospital you called home? Your were a young child he decided to use as experiments to test human determination in said world and breaks physical laws to test human possibility? He knows of the powers you gained from his experiments... all but one, this one allows you to tear realities just enough to teleport into them. Knowing of Undertale you felt bad for the monsters stuck below in their world and wish to help them, but something goes wrong and you end up going to certain AUs and helping them, gaining the attention of some bad skeletons who now desire your power for them-selfish-selfs.