But you can't spell determination without termination. Plus we didn't see any of the human souls change colour. The blue and aqua souls definitely killed. Plus we don't see Frisk's soul change during a genocide run, so a red soul is better than a black soul. Noone is good or evil when they're born so having a soul that would mean ' omg what a freakin weirdo, definite future serial killer here' confuses me.