You never really trusted them; Toriel and the others. After all if they really loved you they'd let you go, they'd let you fight for yourself. Yet that stupid goat had to try and 'protect' you didn't she? She had to go and stand in your way, treat you as if you were just another fragile human. What happened was her own fault. The monsters of this world should have known better,
When it all started you didn't truly intend on killing them all. You didn't want to waste your time on them; they forced you. They stopped you in your tracks, they slowed your progress. So cheery in their tomb trying to make you like them, a happy little prisoner. You couldn't have that. You made it your mission to set them free for once and for all, every single one. You'd make sure that one way another the underground would go empty.
By the time you came face to face with Undyne there was really no stopping. She wouldn't have allowed you to simply quit. Quite a noble heroine she was, given half a chance she would have surely killed you. So you made sure not to give her even a quarter of one. It was just like Flowey said: It's kill or be killed. You liked him. He understood. Such a shame he ended up as weak as the others.
Then there was Sans, oh Sans. He knew you well. He could see straight through your power: mocked it, challenged it, even used it against you. You were almost proud of him. You remember thinking how powerful a pair you would have made if he had only picked the right team. Of course no matter how many times you faced him he never did. Nobody ever does. He fought hard, to hard in the end. It was a cheap victory but there's no extra points for playing fair. Every fight must have a victor, how it happens is no concern of yours.
This emptiness wasn't truly your doing. You just delivered the final blow to an already dying world. You've fought everyone there is to fight, you've destroyed everything there is to destroy. This world trembled and fell through your might. It was so easy. There was only one or two occasions where you even had to try. Too easy, too easy. You want more. You step into the darkness once again awaiting a challenger. There has to be more. You want it harder, faster, better. You wait forever for a worthy opponent, an entirety for that thrill.
But nobody came...