Jury is almost certainly hang up on a political matter. Acquitting Kyle would mean you can indeed shoot a motherfucker, even as a 17 y/o white boy (at least in Wisconsin).
And that's a problem. It's obvious that self-defense is justified in all of the charges and he is not guilty. It's not just potential riot and whatnot, the implication is far bigger than that and they know it. No lefty ideologue would want to hand the right this W, even if it means dooming an innocent young man to rot in prison.