I wouldn't say it's clear-cut, but if he was the instigator (pointing his gun at people or making verbal threats) then that significantly weakens any self-defense claim.You should read this:
https://docs.legis.wisconsin.gov/statutes/statutes/939/III/48
Any decent prosecutor will get the conviction. The psyco kid will go to jail for a long time.
The law does permit the use of lethal force, if you believe you're in danger of great bodily harm or death, and if you have exhausted all other options to avoid it. Given that someone fired a warning shot, a reasonable claim can be made that he thought the first victim, who was running after him, was the one who fired the shot, thus there's room to claim he thought he was going to die
The second incident, where he killed one and injured another, he had several people bearing down on him (one kicked him, another hit him with a skateboard, while a third had a handgun), so again room to claim he was fearing for his life.
It sickens me that this little fascist shit even has a chance to snake out of this, because I'm convinced he was hoping to get to shoot someone.