Doing so is often done by players who complete a No Mercy/Genocide run who want to regain the True Pacifist ending. In musical form, of course, such a device is not really feasible which is where Gaster comes in. Gaster is depicted as simultaneously well-intentioned and arrogant, with lyrics implying he has become something more than a mere monster. In this case, he represents players who, while well-intentioned enough to want the True Pacifist ending, wanted to still get to 100% completion through a Genocide route and are willing to in essence warp the game's reality to have their cake and eat it too because they are above consequences.
This also is why it's not on the full Genocide playlist-not all players actually try to avert the consequences of their actions this way.
