If this happened as is described here, it is 100% justifiable. And if it happened that way, one can only speculate on why a young black man would challenge police in that way; did he believe they actually wouldn't shoot? Did he believe law enforcement in the St. Louis area has been intimidated into inaction when threatened with a weapon, including a knife? Did he want to die, to create a new flash point in the area?
Photo credit: By Dmitry G (DSC00061 Uploaded by Partyzan_XXI) [CC-BY-SA-2.0 (http://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons