While the vocalist alternates between singing on the ground and singing in the air, a bunch of bosses you murder-death-killed earlier in the game act as her band, because this ending would have made no sense if it was just a girl singing with nothing around her to make the music.
As she keeps singing and flying and n'a change-ing like she just don't care, Hectic begins to slowly go crazy. So utter destruction and hundreds of dead bodies don't faze him, but a pop star with crazy eyes, warbling words he does not understand, is what brings out the PTSD.
The German version of "99 Red Balloons" once made him burn down a school.
Then, just to give Hectic one more reason to scream, the alien band comes back to play again, all while scenes of their deaths play like a Vietnam flashback. Finally, after what feels like 500 years but was actually just over three minutes, Hectic collapses in a heap, the song begins to skip, the screen fades to black, and we're left to stare blankly at our television and wonder if our lives have meaning.