Rewatching The Wicker Man (1973), it's really striking to me how much the ritual is for Lord Summerisle's own benefit as much as anything else. In fact, I would argue that, in some sense, the sacrifice is for and to him (as he himself blurs the distinction in Aphrodite's case).
Just as the ritual is meant to appease both Nuada and Avellenau, both gods and men, both Lord Summerisle's hedonistic thirst for life and the people of the island's hunger for both produce and worship (Summerisle the man, the people of Summerisle, and the island of Summerisle itself), so too I think Lord Summerisle's motivation is double: both expediency and love. The film leaves it ambiguous whether he believes or not in what he preaches, and I think he does and does not. As a politician and a zealot, he believes in the way a really good actor has to believe in what he's doing, and he's both. Because he loves the drama of it most of all, I'd say, and the difference between a ritual and a play is so thin.
He loses himself in it! He does, and he looks so thrilled when he's singing and burning Howie. His hair flares out in a corona and he's wearing yellow and his head overlaps with the sun in one shot. The sacrifice is for himself because he's the sun.