Depressed after seeing video of girl drowning

I seen on the web a video from a security camera taken September 1st in the Dominican Republic. It is of a 15-year old girl being pushed into a swimming pool by a 16-year old boy. She couldn't swim and was drowning. No lifeguards were around. There were many people in and around the pool but no one noticed she was drowing. I know they say it's difficult to tell if a person is drowning but I could see her obviously struggling to surface for air and constantly sinking below the surface. She tries desperately to go towards the edge of the pool while underwater. Then the boy who pushed her in grabs her and pushes her deeper underwater. She then sinks to the bottom while thrashing her arms to no avail. It isn't until over 10 minutes later that someone noticed her at the bottom of the pool and pulled her limp body out. They tried CPR but it was too late. That piece of garbage boy has since been arrested and prosecutors are trying to charge him as an adult. I've seen many disturbing videos but this one really made me depressed.
Why would someone do that to such a pretty young girl?