Board Thread:Questions and Answers/@comment-43517151-20190815231711/@comment-26907577-20190816011959

You don't need an in-game explanation. It doesn't exist. The player has Frisk commit genocide because of whyever they want to. Possibilities:

-Legitimately want to destroy the world of Undertale. -Want to make a Let's Play with this particular ending. -Want to get the "full experience" -Want to fight Undyne the Undying and Sans. -Want to record all the possible dialogue for this wiki.

The thing is, all of these are true, but which one depends on the specific player. Different characters come up with different reasons:

-Chara thinks it's the first. -Flowey thinks it's the first, but references the second. -Sans concludes the third during his fight.

Which one is correct? That depends on the player.