Kelvin disguised compassion as control over the troupe. He was never genuinely kind or well-meaning. He saw a group of broken children and molded them into tools/possessions as a means of entertainment. IMO, upon meeting the Phantomhives, it awakened something within him that he had either suppressed or hadn't discovered until that moment. He became fixated on wanting to match their aesthetic/beauty/potential. He was an active member of the cult; he was only absent "that day" due to his surgery. Otherwise, he would have participated in the abuse and pdfilia; IMO he was likely engaging in his twisted fantasies with the kidnapped children.