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

While this does hold true in general, we also must understand that it is not always the case.

Imagine you're a lion and you're hunting gazelles. Would it be easier for you to hunt a male gazelle in its prime, when its is running at top speed? Or is it easier for you to hunt an older or younger gazelle as neither of the two can run fast.

While you might most often decide for the latter two, sometimes, it is also possible you will hunt a gazelle that may be more chalenging to hunt but is the best choice as it is the only available one in the area.