What was this guy doing on the streets?

Casimiro Barros was an "armed career criminal" (quite an accomplishment for someone at the tender age of 20). He'd been arrested most recently in December for unlawful possession of a firearm - after he'd led police on a high-speed chase. And yet now he's charged (along with another alleged winner) in Chiara Levin's murder. Why wasn't he behind bars on the earlier charges? How many other ticking time bombs like him are walking around today?