It is I, the Barn Owl Man

like spiders crawling on your back

My KillerDust take

After many runs of Undertale, the Player finds that on a Genocide Route, someone had already started killing Monsters. This being is called Executor/Executioner which is a being similair to the Player with the goal of ending the world.

My DustKiller take

"Murderer" is the result of the Player doing multiple Genocides and growing bored of Undertale, causing Gaster to implant them Snowgrave style into Sans to experience something new. Sans isn't concious while this is happening.

That Dusttale take again

It now has Deltaslop, as Sans has a Shadow Crystal (specifically in his damaged eye socket)

That's why he knows

The Ramb