Let's be real, this is simply the reverse of all the people who dismissed the treatment of George Floyd because he'd done crimes in the past, dismissed the treatment of Michael Brown because he'd done crimes in the past.... etc. etc. etc. etc. etc. etc. etc. And trust me, search my handle and you'll find I'm no leftist. I just don't like double standards and hypocrisy.
I also work for a prosecutor's office and while this is an unusually brutal assault, I wouldn't call it strange to charge a 15-year-old as a juvenile in an assault case -- I assume that is the charge, and not attempted murder or something. I'm in a different jurisdiction but usually our attorneys won't even consider certifying a defendant as an adult if they aren't at least 16 unless it is murder or attempted murder.