Well for Hiroki at least it makes sense that he doesn't kill the king. If he do stays in this world, killing the king will most likely make him a forever wanted person, at least in this kingdom and the others that are allied to it (don't remember if there is any

).
Because being real here, even if Hiroki has all the rights to kill him, the king
is a much more important figure in this world's society than him, and realistically his son (prince) would be pressed by the kingdom nobles to hunt Hiroki down, because what nobles wouldn't use that opportunity to say that a prince, now king, that don't retaliate to his father killer is weak and an unfit ruler?
Lusha would be made a forever fugitive too alongside Hiroki, they could verry well flee to Demon King realm but his friends journey for the remaining keys and secrets about summoning magic would be a dozen of times harder with the killing of the king.
And the Hero and co. could very well be just acting humble and meek to gain what they do want (less interference in their actions, as opposed if they kill the king),
there is a thing called lying 
So they pretend they're lowering their heads now so they can have less trouble finding the remaining keys and getting the fuck out of this world.
I know senseless retaliation is what most people here want to see because they are trying to vent their irl frustrations and relax, but sadly this isn't always the case in real scenarios.
Ask yourself: What are the chances the rebels (and killers of the ruler) of a previous government will be left at peace when a new party takes over?