They probably DID hear voices in their head.
Drug induced schitzophrenia is like that.
The VAST majority (all of them except for Islamic Terrorists.) were taking mind altering drugs at some point in their lives. The drugs are sold to vast numbers of people.

sertraline (Zoloft)
fluoxetine (Prozac, Sarafem)
citalopram (Celexa)
escitalopram (Lexapro)
paroxetine (Paxil, Pexeva, Brisdelle)
fluvoxamine (Luvox)