In the very beginning of the movie, Kull (Hercules) is attempting to join an army, but fails. This is odd, because he seems to be a foreigner since he seems to know nothing of the local culture. Therefore, why would he try to join an army of a country he's never heard of? The King has gone mad and is killing all his sons, and Kull stops him by killing him. The army I already mentioned were all made up of men of royal blood (I.e. the King's relatives) so, since all the Kings direct heirs are dead, there is some confusion as to which one of the soldiers is the rightful heir. It's most likely one of two of the soldiers, and of course they themselves are somewhat bias as to which one it is. However, the King, with his dying breath, decrees that Kull is the next King.
And that's the first three minutes of the movie. (Or might have even been the first MINUTE).
This is basically a low-budget sword-and-sorcery movie. The culture isn't established enough, Nothing's really explained about Kull's motives other than liking axes and not liking slavery, and basically the whole movie is alright but not great. See this movie if you just wanna see some low-budget sword-and-sorcery, or if you like to watch movies for the purpose of laughing at them. If you like GOOD movies, I suggest Lord of the Rings.