Doubt what? How a witness is discredited is not the issue. The issue is getting a jury to not believe a witness's recounting of the event. The accuracy of the witness's account is irrelavent. Plant any doubt in the mind of the jury re the witness's capacity to be truthful or reliable and the defense has a far greater chance to be successful. Discovering the truth is a noble cause but the truth is neither friend nor foe. If there is a chance that the truth is the defenses' foe then the truth can be sacrificed. Or, a excuse (justification) is sought to mitigate the defendant's actions. It appears that the defense is seeking a excuse (justification) to the defendant's acts.
Hyperbole: "Yes, my client did shoot the decedant, this is fact. He was justified because he wanted to protect the public."
Whether you intended to or not, you have hit precisely the point.
There are only two (2) exceptions to other than a guilty verdict: excuseable or justified.
http://www.self-defender.net/virginia-self-defense-laws.htm
http://www.virginia1774.org/Dodson.html
It is therefore most consistant that Mr. Hawes would persue one of these in defense of his client.