I… I’m watching Jack’s playthrough of Undertale and I just realized exactly what Toriel meant.  She says exactly this:

She doesn’t say: “Killed six humans” she specifically says “TAKEN from the humans”.  ALL human souls persist after death.  So this probably means that in a world with only humans and a lot of death, he could probably just take souls from graves.  And also free the monsters without inciting a literal war.