Killing with Kindness (1) (S01E03)

A series of patients in the same hospital ward perish within moments of one another. Upon investigation, it is revealed that they all suffered from a drug overdose, and each bears the tattoo 'let me go' on their backs. Creegan and Taylor turn their attention to Carl Burgess, the hospital cleaner, who is discovered with belongings belonging to a prominent doctor. The situation escalates when Creegan is assaulted at home and left unconscious. As the mystery deepens, the question remains: who is behind these tragic events?