...Well it's based on the idea that building up your own Determination would awaken the primal spirit that is Determination.
Determination is self-aware, understanding perfectly that neither itself, Chara, or Frisk is to blame for the genocide that is occuring, but the Player.
It chooses to punish the player after their inevitable failure, both murdering them and forever corrupting their game to remind them of their sins even if they try to fix things.