This is the 3307th most frequent French word.
tueur
Killer.
The word 'tueur' here refers to a person who commits the act of killing, commonly used in a crime or murder context.
Le tueur a été arrêté par la police.
The killer was arrested by the police.
In this usage, 'tueur' is part of the noun phrase 'tueur en série', meaning 'serial killer', which is a specific type of killer.
Ce film sur un tueur en série est vraiment captivant.
This movie about a serial killer is really captivating.
Here, 'tueur' is used metaphorically to describe someone who is extremely effective or aggressive, not literally.