Protection

Rossi: "It is a man's own mind, not his enemy or foe, that lures him to evil ways." Buddha.

An UnSub is egged on to kill people by hallucinations of his childhood friends, who died in a fire he started.

Tropes in this episode

Hallucinations: The UnSub hallucinates the people he lived with before he killed them as though they are still alive.
Through the Eyes of Madness: The UnSub suffers from hallucinations and believes certain things are happening. During one of the murders, he believes the surviving woman was being attacked and was egging him on to beat and kill her “attacker”. It turns out they were a couple and the girlfriend was begging him to leave her boyfriend alone.
Vigilante Man: The UnSub is a murderous variant.
Wrong Assumption: The Unsub sees a man grabbing at an elderly woman's purse and assumes he's mugging her. In reality, they were close friends, and he was trying to grab her epi-pen while she was having a seizure.

Reid: "Reality is merely an illusion, albeit a very persistent one." Albert Einstein.