essentially, i believe megalovania is chara's theme,

because in Undertale's Genocide, sans tries to kill u not other way around

whenever you fight a monster, like undyne or papyrus, their theme plays

which means that you, or chara,'s theme would be playing

also the song's name doesn't fit sans at all, it fits chara

megalomaniac: a person who has an obsessive desire for power

that's just the base though

im gonna be deciphering the song itself

first line, that d4 d4 d5 part stands for the hopes and dreams of the monsters of the underground of getting to the surface similar to asriel, chara was a hope for humans and monsters to unite

that next part, g#4 g4 f4 d4 f4 g4, stands for the genocide route. one by one, Toriel, Papyrus, Undyne, Muffet, Mettaton and essentially Alphys are killed

asgore isn't in there he's sipping tea

(afk for now i'll come back later)

the entire a4 g4 f#4 f4 g4 stands for The Ruins, Snowdin, Waterfall, Hotlands, and the Palace. With each step, the player descends further into darkness.

c5 c5 c5 c5 c5 c5 a5 a5 the c5s are basically the hope of the underground as they sink lower essentially the "g#4 g4 f4 d4 f4 g4" repeated

c5 c5 c5 d6 eb6 d6 a5 c5 d6 dunno, variation of last line

c5 c5 c5 d6 eb6 e6 g6 e6 a6 a6 a6 e6 a6 g6 symbolizes chara's hope as they kill of the monsters

g6 e6 is probably a pang of regret of maybe frisk trying to break free