Board Thread:General Discussion/@comment-26006155-20190617222636/@comment-26006155-20190729155440

Why would Sans kill Gaster?

To save the monsters.

Whether or not The Dark World was a resulting of Gaster's Darkest Experiment, Gaster still needed a way to break the barrier. And the only method that would work was SOUL power, specifically collected from monster SOULs, the entire reason Gaster researched Determination to begin with, so they could be collected.

There's no way that the monsters could be on the surface, unless Gaster found a way to destroy the Barrier. It would take a massive amount of power to take the soul from the living monster.. which is why Gaster designed the massively powerful Core to generate the powered needed to do this.

Say the fight between Gaster and Sans happened, similar to how I described it in Chapter 9. Then Sans went back in time to try and save Gaster.. only to realized that he, Dr. Andonuts, had been corrupted by Giygas long before they even traveled to the Underground.

Sans/Ness had one mission in life: To defeat Giygas. The very entity now possessing a person he once thought was a friend.

Gaster was super-popular, and he'd decieved the monsters into buildilng their very own deathtrap in The Core, which would break the barrier by sucking in all their SOULs to provide the needed power.

Gaster was corrupted, and Sans couldn't reason with him. And he couldn't get anyone else other than Papyrus to listen to him about Gaster's dire plans to destroy monsterkind before they could destroy humanity when the Barrier dropped.

So when Sans went back in time.. perhaps 10 years back in time just like Ness did?... he not only killed Gaster, but erased the memory of him from the Beta Timeline.

If Gaster's ghost is indeed controlling Hyperdeath Asriel, and he's looking to punish Frisk for what his future self Sans did to him earlier... it makes 100% sense why Gaster would make erasing Frisk's memory from the world such a deliciously ironic fate.