Why haven't they arrested the former employee?
You have your facts completely mixed up. The man falsely accused by Reddit, not the FBI, had died a month before the Boston bombings occurred. The entire reason he was accused by Reddit is because he was already known to have been missing. His body was subsequently discovered more than a month after he died.
The guy he's talking about was already dead weeks before the Boston bombing occurred. Reddit could not have been responsible for his death, reddit libeled a dead man.