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?

The only moral response is “defeat the demon king on your own immediately, saving a sizeable number of people in the process,.

Anything less would be immoral. Morality is more important than somebody’s chance to be a hero. They can go be a hero somewhere else.

Yes, I would kill baby Hitler.

Is it OK to get some details?
If so, is the hero at least no worse than the demon king?
Is the hero a good looking female, or a guy?
Am I permitted to do contract on behalf of the hero?

@josie
It is a general question.

My choice would depend on a few considerations that depend on other details.

1) How certain am I that I know what the larger situation is? Am I sure I’m the only superpower here, or if I intervene, is it possible that other hidden superpowers may notice and also intervene. And is there some other annoying deity that put me here which has some annoying agenda standing in judgment or trying to teach me a lesson or something? Apprehension of bigger concerns such as these may have me be more cautious and less interventionist. If I’m sure it’s just me, then I’m far more likely to shape things to my own tastes.

2) Are there limits to how much or how many times I can intervene? If so, I might tend to choose to conserve my resources for later/other/unknown situations. I might also try to cleverly influence things with my knowledge and information rather than overt use of power.

3) What’s my assessment of the specific results of the known plotline? What do I think would happen if I intervene? If I think the world will be a better place with intervention, then I’d tend to intervene. If the known end situation seems better on balance despite the death & destruction, then not. Might have a lot to do with who died. e.g. in Game Of Thrones many of the deaths are of people whom the world might be better for them to be dead.

In general, unless it’s a great story with some advantages for the specific way it turns out, I probably won’t be very attached to story. In many stories I find some turning points where what happened irks me in one way or another, which are likely places I’d be tempted to intervene. So yeah, I’m likely to intervene, but still carefully and cleverly in preference to relying on power. Foreknowledge is often power enough to turn around situations.

That being established, I would stay out of it. Lots of heroes become demon kings.

Does the ‘Prime Directive’ not apply in this fictional/real world?

Kill the baddie. The hero can find some other lame quest to Leroy his way through.

Defeat the demon king straightaway. Allow the protagonist to take the credit since such shamefully little effort is required from me.

Simple, I appear to the Demon King (like Jesus purportedly appeared to Constantine) cause his conversion to my way (like Constantine converted)... and then sit back and watch the real carnage unfold for centuries to come.

I’m sure you must be looking for inspiration for book writing. Well, if granted such power and knowledge what could stop me from becoming the protagonist of the story itself? That’ll have to depend on what power I have. I’ll like the latent power (that I accidentally awakened )to tame the demon king so that he’ll have a change of heart, he’ll show the good side of a demon for the first time ever, our understanding transform in to bittersweet friendship, friendship blossoms in to love, and love gives birth to courtship and union. Oh my, such drama! I think I might have transformed the story in to romance genre.

Forget about the protagonist. The Demon King is a lot hotter and more satisfying in bed. On the second thought, a love triangle sparked by a jealous protagonist would also be interesting.