Cause you can't be a well known face in the community and go on a killing spree without at least some of the killings being corroborated.
I could see if the claims were that he was an underground serial killer stashing bodies around the city. But 15 people don't just end up dead in one day without anyone noticing. Had he really killed 15 people yesterday, we wouldn't have even needed his video to tell us.
He didn't claim to kill 15 people yesterday or in one day in the video, he just claimed to have bodies in abandoned buildings on some want to be The Wire shyt. Dude is obviously crazy from a mental health stand point, until the police clears up those claims I don't think its absurd to think this dude could have been some kind of serial killer for years now, randomly killing people when he got the urge to do so like he did that old man yesterday.