Hopefully this explanation is allowed to provide context to the question.

I don't know the historical context of this case (other than what I just googled), but from a legal perspective I believe there are two major misconceptions confusing you. (1) Self-defense does not require the aggressor to pull a weapon first, and (2) within fairly minor limitations, US juries have broad discretion to give the wrong answer (either because of bias or on purpose).

For the first point, somebody can threaten your safety enough to justify self-defense even if they don't pull a gun. If they do so, and then they pull a gun after you defend yourself, that fact is irrelevant to the legal status of your behavior at the time of the purported self-defense (other than, arguably, that it proves you were right to believe they were armed). It is a question of what exactly Slough said and did and did that justify self-defense under the applicable laws to the standard of proof required in that jurisdiction at that time. Maybe the answer is no, but that won't necessarily just be because he pulled his gun after getting shot. The general standard is whether a reasonable person would be in fear for their life at the time of the shooting. Some states don't require any attempt to escape first, and the burden might be on the prosecutor to prove that it didn't happen. The shooter might only need enough evidence of being threatened that the prosecutor can't prove he wasn't (this depends heavily on the specific laws and standards in question).

For the second point, I'll note how subjective the first point is. All you need to do is convince the jury of a subjective feeling, really. Sure, it's supposed to be an objective standard about a hypothetical, perfectly reasonable person, but that's not how humans work. If the jury wants to let someone off the hook, or just naturally empathizes with them, it is easy for them to justify a finding of not guilty and there is almost never any recourse regardless of how wrong they are. They can even decide to ignore the law on purpose and there's almost never any recourse.

Sometimes a judge can stop defendants from even raising a certain defense if there's literally no evidence supporting it, but most of the time defendants get to make their argument. So at that point it's not even a question of whether it was self-defense in fact - it's a question of whether the jury is willing to say it was.

This is still a huge issue in America, with regular debates about how our internal bias can change our perception of whether behavior was "reasonable," based on whether we empathize more with the shooter or the person who was shot ("our" being the police, prosecutor, judge, jury, media, and public).