Those who want to discover all facets of the story must endure the Undertale Genocide path. If you’re kind-hearted by nature, it would be an almost unbearable challenge. You must stab every single soul involved in the fight, become a silent murderer deaf to pleads. You will have heavy conversations with those whose friends you’ve killed. Things become more complicated when Sans and Flowey remind you of your former peaceful saves.
This route makes you bear the consequences of all actions. But you should pass at least a small part to get a full understanding of the story.