Much of the pagan stuff looks to be based on James Frazier's work, which has been criticized as inaccurate. Considering that the pagan cult was made up by the original Lord Summerisle, who belonged to the Victorian Era, that source would indeed have been the best work on paganism available to him at the time.
Some of the actions of the islanders become more clear after it's revealed that virtually everything was part of the plan:
The townsfolk almost immediately begin teasing Howie about being seduced by Willow, and she propositions him soon afterwards. Howie's sacrifice was contingent upon him being a virgin, so this was a test to confirm his status. The original script made this explicit in dialogue.
Willow and MacGregor make a rather clumsy attempt to "immobilize" Howie during the May Day celebration, but it was already heavily implied that the locals sabotaged his plane to keep him there. Their motives therefore make no sense until it is later revealed that everything was part of the plan: The Hand of Glory was entirely a ruse to prod Howie into joining the May Day festival.
Lord Summerisle chastises "Punch" for not playing his role correctly and gives him instructions, but he does not seem suspicious even though MacGregor plays this role every year. But Summerisle already knows that Howie is in the Punch costume, so Summerisle is simply preventing Howie from getting suspicious and wondering why no one is noticing that he does not know what to do.
The restored edition adds a lot of motivation for Howie. Not only is he engaged, but his fellow police officers mock his virginity constantly as well as his faith. The villagers are pressing all of his Berserk Buttons, and the only reason he is able to withstand Willow's seduction is because he is engaged, not because he is a virgin and/or religious.
Howie discovers that a schoolchild has a beetle tied to a screw and mocks how it spins round and around, always the same way, to its destruction. Howie chastises her for being cruel. The beetle represents Howie himself, who has been lured to Summerisle and led around a predefined path to reach his sacrifice. As a Genius Bonus, beetle is a homophone for "beadle," an old term for a policeman.
Howie warns Summerisle that if the crops fail again, he will be the sacrifice for next year. It is likely that the crops will continue to fail, and the community will therefore kill its leader and then descend into anarchy as their leaderless society tears itself apart with even more futile murders. They use rituals and magic while trying to solve their very practical problems.
Summerisle notes that a child is a good sacrifice, but an adult virgin is better. It is possible that they really have already sacrificed at least one child, as Howie feared had happened (or was going to happen) to Rowan. If the plan had failed with Howie, they might have gone ahead with sacrificing one of their children, perhaps even Rowan herself, in his place.
Howie mentioned having a fiancée back home. And Summerisle also states that he and the townspeople will make sure his disappearance is explained. Just imagine the confusion and other mixed-up emotions Howie's fiancée might feel from whatever lie they fabricate.
