The Six Human SOULs are the remains of the six humans who previously fell underground and were killed by monsters. They were collected as per order of Asgore Dreemurr, to allow him to finally break the barrier, freeing monsters from their prison. If Frisk has killed everyone and let Chara takes control, Alphys will have called Asgore in an attempt to get him to absorb the SOULs and fight back. However, it seems Asgore failed to heed this warning, as Chara entered the castle and effortlessly murdered both Flowey and Asgore.