For wild animals, every day is a struggle. That struggle for the most part happens out of view of humans. Since the invention of trail cameras, we have been able to get to see the tough world that nature faces every day.
In this case, a deer attempts to get a drink from a watering trough but has no idea that a mountain lion is lurking nearby. It does not take long for the mountain lion to make its final approach and pounce.
The actual attack takes mere seconds but the deer’s struggle for life takes a lot longer. Being killed by a predator is a rough way to go. Hunters do their best to make quick clean kills with as little suffering as possible, nature usually is not so kind.