Yeah, that is pretty obvious.
You may want to actually read the law you previously posted.
I read it carefully. The correct charge for an adult would have been 2nd degree homicide. The legal issue is proportionality of the response to the threat.
Wisconsin law says, paraphrasing, force can only be used to the extent reasonably necessary to deter an attack. Lethal force can only be used if one is about to die or suffer a devastating injury.
Also:
Mitigating circumstances. The following are affirmative defenses to prosecution under this section which mitigate the offense to 2nd-degree intentional homicide under s. 940.05:
(b) Unnecessary defensive force. Death was caused because the actor believed he or she or another was in imminent danger of death or great bodily harm and that the force used was necessary to defend the endangered person, if either belief was unreasonable.
If Kyle believed lethal force was neccessary; 1st degree was out. Then the issue becomes whether lethal force was reasonable or not for "a person of ordinary intelligence and prudence in the same situation"