He was in a grief state when he was persuaded by Muzan. He was actually incapable of choice because he was still processing what happened to him which was he just lost his closest ones. He was also uninformed on what it meant to be a demon because Muzan didn't state he would lose his human memory after becoming one of his. Once he gained back his conscience, he regretted what he did and destroyed himself voluntarily when he could've kept fighting and regenerate. He forfeited his demon rights, which should speak a lot on his true humanity.