To understand how the castle law works, it is important to understand the underlying laws for self-defense and the use of force related to protecting others and property. The key word in all of these laws is “reasonable.” For example, it is never considered reasonable to use force against someone for words alone. Tex. Penal Code § 9.31(b)(1). If the force used is reasonable in the situation, then using force is considered justified.
When force is used in the specific circumstances of the castle doctrine, the law presumes that the use of force was reasonable. The presumption of reasonableness makes it much easier to avoid criminal charges, and even if you are charged, it is much easier to prove that your actions were justified.
Texas Castle Law Doctrine | Self Defense
Sec. 9.31. SELF-DEFENSE.
(b) The use of force against another is
not justified:
(1)
in response to verbal provocation alone;
(2) to resist an arrest or search that the actor knows is being made by a peace officer, or by a person acting in a peace officer's presence and at his direction, even though the arrest or search is unlawful, unless the resistance is justified under Subsection (c);
(3) if the actor consented to the exact force used or attempted by the other;
(4)
if the actor provoked the other's use or attempted use of unlawful force, unless:
(A) the actor abandons the encounter, or clearly communicates to the other his intent to do so reasonably believing he cannot safely abandon the encounter; and
(B) the other nevertheless continues or attempts to use unlawful force against the actor;
Sec. 9.32. DEADLY FORCE IN DEFENSE OF PERSON. (a) A person is
justified in using deadly force against another:
(1) if the actor would be justified in using force against the other under Section 9.31; and
(2)
when and to the degree the actor reasonably believes the deadly force is immediately necessary:
(A) to protect the actor against the other's use or attempted use of unlawful deadly force;
PENAL CODE CHAPTER 9. JUSTIFICATION EXCLUDING CRIMINAL RESPONSIBILITY
IMO, KA provoked ( brought the knife, seated himself where he knew he was unwelcome, issued the "touch me and see what happens" taunt) the conflict with his victim. KA used deadly force in response to what was reported to be a shove by the victim, to which he responded "punch me, see what happens" (further escalating the situation), on receiving another shove/push, KA drew his knife from his backpack and stabbed the victim.
None of Autin's described actions (telling KA to leave, pushing/shoving KA as he was being encouraged to do by the accused's own words) warranted deadly force in response. KA provoked the conflict, escalated the conflict, and then murdered AM.
IMO it matters not one bit if he knew the "unspoken" rule or not. Deadly force is not required in response to a shove or two or even three. A couple of shoves after basically being dared to do so does not an imminent threat make.
ETA

that's what I get for taking a call while in the middle of writing this post and not refreshing before finishing and posting.
@Seattle1 basically said what I was thinking on this topic