Nobody really knows why some people do this, but it is something that is most likely true. I don't know if anyone has been caught doing this, but it may be happening when we don't realize. Maybe they want to look back on what they accomplished, but it seems very cruel. People need to stop it, that is for sure.
Yes, some (dumb) criminals use cell phones to capture crimes they commit. Many people have an obsession with being popular. Social media has fueled this desire for popularity by inspiring some very dumb things sometimes. When criminals capture their crimes on cell phone cameras and post these videos to social media, the public and law enforcement can track down these criminals much more easily.
Criminals have never been known for their intelligence. There are times that these criminals even video themselves committing the crime. They want to be able to brag to their friends later or relieve their bad deed. Often times, though, this doesn't work out for the criminal, and the video ends up as evidence.
We live in a very narcissistic society. This has been influenced by the onslaught of technology that has given us the opportunity to document every aspect of our life. That is not limited to illegal activities either. On top of the narcissism, we are also living in very ignorant times where people will document their crimes and post them on social media sites just to get famous or rack up the street cred. The world is really getting ridiculous.
A normal person will not capture a crime he is committing on a mobile phone. This is an act that can be done by persons with mental problem, because surely, who would want to be found by police and taken to jail. Mobile phones can be hacked and its contents accessed, so it is a risky affair.