Is the novel against dictatorship? But why? I think a very strict government is the most efficient way to restrict the inner beast.
Here's a quote from William Golding:
The theme (of Lord of the Flies) is an attempt to trace the defects of society back to the defects of human nature. The moral is that the shape of society must depend on the ethical nature of the individual and not on any political system however apparently logical or respectable.
So yes, Golding's novel is against dictatorship (as evidenced by Jack's ruling of the island, and the brutal chaos it begins - though the precise nature of the violence, with all its sticks sharpened at both ends, is left elliptical).
And, though many right-wing critics would agree with your assertion that "a very strict government" is the best way to hold back humanity's "inner beast", Golding doesn't. The argument he's trying to make that it is a society full of good individuals that becomes a good society. That you can't impose goodness through rules. You have to foster it from within. And the society in which "Lord of the Flies" is located is set in a war - the plane is "shot down", and at the end, its a naval officer who rescues. The sort of behaviour in the book, Golding argues, is what a society at war breeds.