Apparently he was some kind of wannabe gangster rapper.
There's another post here on KiA2 linking to a NY Post article.

The “person of interest” arrested in the deadly mass shooting at a Fourth of July parade outside Chicago on Monday has been identified as a 22-year-old local amateur rapper with a history of disturbing online posts.
Robert “Bobby” Crimo III, who performs as “Awake the Rapper,” posted a video to his since-deleted YouTube page that included an image of a man with a long gun face down in an apparent pool of blood with a few police officers standing in front of him, according to screenshots posted to social media.