Do predators really kill only the old and sick prey? What evidence is there for this statement

There are many factors that determine what a predator will prey on. For instance if the predator is starving or if they are hunting in a pack. Predators will usually prey on animals that are easiest to be captured without using too much energy. The sick, young, old, small and unaware animals will often times fall prey since they are easiest to be caught. Predators don't necessarily prey only on the ill / injured. Sick prey may be overlooked because their bodies may not provide the necessary sustence for the predator and others it may be feeding. Healthy large slow moving animals may be difficult to kill but the payoff is greater, especially if a pack is involved.