Board Thread:Questions and Answers/@comment-7371650-20160705050546/@comment-27701762-20160705053824

No, there's no reason, if you know what to do and that you specifically want to get the True Pacifist ending as quickly as possible, to kill anything on your first playthrough. The design and intent of the game is that you should hopefully go in not knowing this.

However, there are some dialogue changes if you replay certain sections, so you could defeat Flowey and then reset the whole game and start from the beginning to see the little changes. But in this case there still isn't any specific reason to kill or not kill anything.

There are some minor changes here and there if you kill stuff, but those are very specific changes, and explaining them may either be trying to inform you of things you already know, or else be potentially spoiling some surprises for you.