Dare

I dare you to kill your main character. Not in the way that they'll come back in the end, but in the way that they're gone, and there's nothing that will bring them back. No plot devices, no time travel, no nothing (even if any such thing exists in your universe).

They can't be bad either - not someone who's secretly working for the other side, or someone whose ideals don't match up with that of the world, or a person whose death is their redemption.