He does not appear to have a "self defense" defense, particularly with the second two shootings as he was a fleeing felon being chased by good people trying to apprehend him for the first killing. See the Wisconsin statute that I posted in reply to Baron.
Although a good trial lawyer will of course sprinkle in self defense, the strongest case is that Kyle was an impressionable well intended 17 year old, who was following his President's "law and order" instructions, reinforced by the Kenosha Police giving him water bottles and thanking Kyle for his armed presence. Hence, the very same Government cannot now prosecute the lad because he panicked amidst the discord.