I just tried facing King and got defeated, opened new place.

But I came to think that what happens if I beat up the king.

Does the game end? or is it impossible to beat king for now?

i wondered the same thing. . . . time to make a grinder game and collect all the resources for top tier gear. but you wouldn't be able to get the matching item without going past that point. . . 

I’m pretty sure you can’t beat him on the first try, something Astaroth says suggests you need new techniques etc that you can only get after meeting him.