RR, like John, I've found that there's little difference in the dynamics of LEO v. private citizen self-defense shootings.
I'm not clear on the cause of your confusion. It's obvious from your posts that you don't have much if any experience in wrongful death claims, criminal or civil, arising out of legitimate self-defense shootings. (I don't do "scumbag versus scumbag" cases.)
Experts are brought in to show things like time-frames and relative danger posed by various weapons, such as knives, to the defendant (TN v. Robert Barnes, for example). Or why a decedent might have been shot many times (TX v. John Allen Curtis). Or why shots striking the decedent in the back could be fired in self-defense (FL v. Mary Hopkin), or why a citizen might be carrying two guns and shoot an unarmed man attempting to disarm him, unless he was a Rambo looking to kill someone (FL v. Zane Britt).
All four of those citizens were freed by the jury after being tried for murder (or, in Mary's case, manslaughter). I was happy to have some small part in helping to regain their freedom.
Prosecutors don't come out of law school with an understanding of the dynamics of violent encounters: speed of fire, disparity of force, action vis-a-vis reaction, etc. They sometimes assume a shot in the back is cowardly murder, that more than one or two shots constitute malice, etc. These things generally require an expert to explain. As you should know as a member of the criminal defense bar, the attorney can't testify, and the client's testimony in this regard is open to challenge as to lack of expertise, and would be seen as self-serving in any case.
RR, if you're a practicing criminal defense attorney, I'm sure you belong to the National Association of Criminal Defense Lawyers, as I do. Check the NACDL archives for the writings of Attorney Lisa Steele. You'll find them enlightening.