Hypothetically, if someone was stabbing Kyle’s mother in the street and he ran back into his house, grabbed a rifle, and [w]ent back and shot the perpetrator would he be guilty of murder because he wasn’t legally entitled to carry a gun?

No, he would not be guilty of murder because he wasn't legally entitled to carry a gun, and Kyle isn't guilty of murder for defending his life against those people attempting to attack him. Both examples are clear cut cases of justifiable homicide while in the defense of one's life or that of anothers.

But in your hypothetical, he would be guilty of possession of a deadly weapon if he were under 18 and didn't have a Hunter Safety certificate.

"But in your hypothetical, he would be guilty of possession of a deadly weapon if he were under 18 and didn't have a Hunter Safety certificate."

Just try to get a jury to convict on that Yo-Yo.

You are in love with legalism but our ancestors came up with "trial by jury of his peers" to address this particular mental disease of judges, bureaucrats, and petty tyrant a55holes.

Educate yourself on your legalism perspective:

legalism noun

le·gal·ism | \ ˈlē-gə-ˌli-zəm \

Definition of legalism

1: strict, literal, or excessive conformity to the law or to a religious or moral codethe institutionalized legalism that restricts free choice

2: a legal term or rule

Examples of legalism in a Sentence

Recent Examples on the Web

His public presentation of special counsel Robert Mueller's report — which detailed appalling wrongdoing on the president's part — was a master class in partisan spin, blurring the underlying misconduct amid a whirlwind of legalism and misdirection.— Star Tribune, 17 Dec. 2020

But does anyone truly think that the public forms its perceptions based on such legalisms and hair-splitting?— Nathan L. Kinard, National Review, 6 Feb. 2020