Some may say then it should be Kings'. But this is not necessarily correct.
The King's World would be possessive of the King.
The Kings' World would be possessive of the Kings.
The Kings World would be a World of Kings, and not possessive. Like a Housewares Convention.
I’d agree with the critique you received here. But it is also difficult to judge the merits of the poster without knowing much about the film itself and the audience it’s intended for. The poster could actually work super well for a very specific niche, while completely failing at the other.
If you want to know more about what it takes to design an effective movie poster, I’d suggest this article