A serial killer kidnapped people and made them take 1 of 2 pills: one was harmless, and the other was poisonous.
Whichever pill a victim took, the serial killer took the other one. The victim took their pill with water and died. The killer survived.
How did the killer always get the harmless pill?
Neither of the pills were poisonous. The poison was in the water that the victims used to swallow their pill.