You have entirely the wrong impression about this. This was a practice run for later life when these cheetahs have to kill to eat. In this case they simply weren't hungry and like many kids "played with their dinner". This has nothing to do with mercy, kindness or any other human trait you might be trying to project onto them. Next time it will be deadly serious.

It is simply that they weren't hungry, that is all. It's better to let their prey live when they aren't hungry than to kill it and let it go to waste.

That Impala knew it was dead, they let it live, a fortunate indecent because in nature the young, the old and the weak are the targets.

In nature you hunt when your hungry, and you leave your prey alone when you don't need food.

Doesn't matter what people say or what the cheetahs are going to do next, the pictures are still cute.