Is it inevitable that Jack will take control of the island?

I think it is. One of Golding's major themes is that human nature, especially in children, naturally gravitate towards personalities which impose their will on others. Most of the boys want to be told what to do with the threat of punishment. This links them with their old world and their comfort zone. They do not comprehend Ralph's concept of shared responsibility. Most of the boys gravitate towards the easiest path where values and punishments are set out for them.