Caveat: I am a lawyer, but I am not your lawyer. The comments that follow are not, and should not be construed as legal advice, but are merely commentary.
With that out of the way, at the Moment of Bang, at least two, and possibly more, legal processes are set in motion. First, there is the possible criminal case. Second, one or more possible civil actions. (One by the Bad Guy, his estate, and if you're unlucky enough to hit an innocent bystander, then possibly one the the Innocent Bystander or his or her estate.)
There are several issues to consider when pondering the advisability of gun modifications. The first is the legal impact of said modifications. For example, having a trigger pull lightened could lead to accusations that it was done recklessly, negligently, or to an unreasonably light level. This is not so much of a concern in the case of an SD shooting where the shooter will likely have to admit that he intentionally shot the BG, but does raise concerns in the negligence context. For example, imagine that SD Shooter draws his gun, and BG McBaddy stops in his tracks. Knowing that he might no longer have BG McB, SD decides to hold off on squeezing the trigger. Through a combination of adrenaline and a lightened trigger, SD accidentally/negligently discharges a round, wounding BG. The argument could then be made that the trigger job was done in a negligent manner, or perhaps that it was negligent of him to carry a firearm that he knew had been modified in such a way. The question to ask, IMHO, is: Does the modification carry a large enough benefit to warrant the additional risk?
The second issue to be considered is one of jury perception. In addition to ay legal arguments that could be made, everyone who carries a firearm should consider what a jury is likely to think about the firearm that is used in a shooting. Is there really any difference between a .223 round fired from an AR and a .223 round fired from a Mini-14 ranch rifle? No. Not to "gun folks." The SD shooter cannot count on the jury being made up of gun folks, though. To many non-gun folks, the gun fired makes a big difference. And you can bet that an anti-gun prosecutor will tell them that they can judge an SD shooter's mindset by the weapon that he carries. If it has a Death's Head carved into the grips, an anti-gun prosecutor will show pictures of it every chance he gets. Again, the question to ask is: Does the modification carry enough benefit to warrant the additional risk?
Sometimes, the answer is yes. Sometimes, it's no. If you decide that the answer is yes, you need to be sure that you can clearly articulate to your lawer exactly what benefits come with the modifications you choose. That way, your lawyer will know exactly what questions to ask if and when you (heaven forbid) ever have to take the stand to defend those modifications.