(Caveat: my opinions are expressed on the basis of Virginia law, and I am not licensed in any state other than Virginia.)
First, I would like to emphasize the same caveat as Dan (quoted above).
Second, just to be clear, I agree with everything Dan said.
Third, I have another caveat: my thoughts are limited to the situation presented. If the facts are changed, the analysis might change, too. The analysis might stay the same. Different rules may apply, or the same rules may have applicable exceptions. Just like Dan, I am willing to discuss hypotheticals at hourly rate. For now, I am limiting my comments to the topic at hand.
Finally...
I would like to add an additional perspective to the original topic. Would having a attorney "on retainer" (or as Dan pointed out, more accurately: "pre-selected") prior to a defensive shooting make a defendant appear to be "trigger happy"? No. Absolutely, unequivocally, no! For one simple reason: it would never be admitted at trial.
Generally speaking, in order for evidence to be admissible at trial, it must pass an initial threshold of relevance. I believe the old phrase is something like: having the slightest tendency to prove or disprove a fact at issue. It's a low threshold, for sure, but it is there, nonetheless. Assuming the evidence is at least somewhat relevant, the judge can/should/will exclude relevant evidence when its probative value is substantially outweighed by its prejudicial effect. So, where the evidence is only slightly probative of a fact at issue, but it is likely to confuse or mislead the jury, or if it is very likely to conjure up an unfounded, general "bad impression" of the defendant, then the judge should exclude the evidence from the trial.
In addition to the rules of evidence, a defendant in this situation would also be protected by his constitutional right to counsel. To explain this, it may be better to give an example with a similar constitutional right, the right against self-incrimination:
Prosecutor to jury: You all heard how he just sat there in the police station and would not explain his story to the officers. And he refused to take the witness stand and explain himself to you, today. What is he afraid of? If he were innocent, he would have nothing to hide. Therefore, he must be guilty.
Defense Counsel to Judge: Your Honor, it is necessary to make my motion for mistrial on the record, or can we all go home now?
Judge: No need counselor. Jurors, thank you for your service. Case dismissed.
To imply a defendant's guilt from his lack of testimony is to completely vitiate his right against self-incrimination. (There is a standard jury instruction explaining that the jury cannot infer anything from a defendant's lack of testimony.) The same holds true for the right to counsel. And, it does not matter
when the defendant retained (or pre-selected) the attorney.
So, here goes: The prosecutor seeks to introduce testimony from someone that the defendant consulted with an attorney a few months prior to the defensive shooting.
Defense: Objection for relevance.
Commonwealth: Judge, the evidence will show that the defendant discussed the laws of self defense with his attorney in anticipation of jumping into the first fight he could find.
Defense: Objection, Judge. That would call for attorney-client communication that is clearly privileged. Even if the privilege was destroyed, it is unfairly prejudicial to the defendant, and it infringes upon his right to counsel. To allow the Commonwealth's implication, the defendant would be punished for simply consulting with an attorney.
Judge: Sustained.
[Note: Depending on the circumstances, if some of this took place in front of a jury, it would open the door to a possible mistrial.]
Any prosecutor worth their salt would never attempt something like this. Unfortunately, there are an isolated few that would try it. However, I seriously doubt that any judge in Virginia would allow it.