Predator & Prey
Predator - Killer
An organism which kills and consumes animals to obtain energy.
Prey - Victim
An animal which is hunted for food by other animals.

The one who is praying (prey) is the victim. The other must therefore be the predator and thus the killer.
Examples
Predators

Lions, sharks and spiders are all examples of predators that hunt other animals for their food.
Prey

Wildebeest, seals and flies are all prey for other animals.