Transported to a previously thought fictional world, with foreknowledge and superior powers, do you feel dutybound to help the protagonist fulfill their quest, or do you feel that you must stay out of the protagonist's story?

For example, the world you are transported to has a demon king on a campaign to conquer the world, laying waste to countless settlements and slaughtering millions of people.
A hero has arisen to defeat the demon king after many hardships.
You are transported to this world with full knowledge on how the story played out previously, and you have been granted power so immense, that you can easily thwart the demon king by yourself.
Do you feel compelled to defeat the demon king on your own immediately, saving a sizeable number of people in the process, or do you decide to do nothing, so the protagonist can complete his story, with the knowledge that many people will die due to the hero’s initial setbacks and difficulties?
Also, there is no indication that the world you have been sent to, is still fictional. It might just as well be real.