Robert Thatcher is one of Dexter Morgan's many victims. He's identified via a blackboard in the background during a meeting about the Bay Harbor Butcher, with Detective Madison being the one covering his case.
Later on, Agent Frank Lundy unzips his body bag in The Tent to reveal a pile of bones, telling Debra Morgan, Detective Ramos and Angel Batista that he's a janitor for a school district...with no criminal records. Debra later confirms that Thatcher was a murderer, but it is never revealed who he killed, why he killed them, or how Dexter found out about it. But since Debra somehow discovered undoubtable proof that he was a murderer, it is safe to assume that he may have had an accomplice who admitted what Thatcher had done.