Board Thread:Questions and Answers/@comment-43517151-20190815231711/@comment-32182236-20190818133809

Frisk and the player are connected far more closely than your typical protagonist is connected with the player.

Undertale's a game about choice. You play the Genocide Route because you choose to.. And Frisk kills because they choose to.

You're roleplaying Frisk. Their reason for killing is the same as yours. And as Jacky points out, one of them could just be to see what happens. That's Flowey's reason for doing it, and there is a parallel between Genocide Frisk and Flowey.

But as Jacky pointed out, it could be for a number of reasons. I consider the first three reasons as all possibilities inside the canon of Undertale.