He wants desperately (if “desperately” isn’t too strong a word for such a taciturn character) to break the mold of his life, and since the resistance won’t have him, he joins the local Gestapo. Now he gets to carry a gun (even a machine gun), and he has money in his pocket. It’s a good job, as jobs go. He doesn’t seem, at first, or even afterward, to have given much thought to the moral issues involved. He doesn’t see himself as a traitor to France, or a collaborator with the evil of Nazism, but as a person of some consequence through his power to order and bully. He likes the work.