Amber Guyger was convicted of murder under Texas Penal Code section 19.02:
A person commits the offense of murder if the person 1) intentionally or knowingly causes the death of an individual or 2) intends to cause serious bodily injury and commits and act clearly dangerous to human life that causes the death of an individual.
The Texas mistake-of-fact defense is codified at Penal Code section 8.02:
It is a defense to prosecution that the actor through mistake formed a reasonable belief about a matter of fact if his mistaken belief negated the kind of culpability required for commission of the offense.
To assert a mistake of fact defense, then, you need to demonstrate that you reasonably believed something that changes whether or not you met the mens rea requirement of intentionally or knowingly causing a death/serious bodily injury.
An example would be firing a gun at what you reasonably thought was a corpse, or a mannequin, or Superman. In any of those circumstances, you would not have intended or known that you were going to cause someone's death. If you reasonably believed your gun was unloaded, you would not have known you were going to kill someone if you pulled the trigger.
In Guyger's case, Whether she was in her apartment or someone else's, she still knew that putting two bullets in a man eating ice cream would cause his death or serious bodily harm.
In the Guyger case, the defendant claimed she was mistaken about what apartment she was in, but that doesn't change whether she had the intent to kill Mr. Jean.
That mistake does implicate the Texas "Castle doctrine" statute, Penal Code section 9.32:
A person is justified in using deadly force against another ... when and to the degree the actor reasonably believes the deadly force is immediately necessary ... to protect the actor against the other's use or attempted use of unlawful deadly force.
...
The actor's belief ... that the deadly force was immediately necessary ... is presumed to be reasonable if the actor ... knew or had reason to believe that the person against whom the deadly force was used ... unlawfully and with force entered ... the actor's occupied habitation
If her mistake was reasonable, that would give her reason to believe Mr. Jean had unlawfully and with force entered her home, making it presumptively reasonable that she used deadly force.
But the mistake of fact defense only looks at mistakes that change whether you committed the offense, not whether an affirmative defense is available to you, so it doesn't really change whether the defense is available.
Nonetheless, it's generally accepted that a person can use self-defense doctrines like this even when they're mistaken about whether they're in danger. The question for the jury in those cases is whether the defendant's mistake was reasonable.
Because the jury convicted her, it necessarily did not believe that she had made a reasonable mistake.
