Wouldn't this same legal argument also be true if you came upon a car crash and tried, but failed, to save the victim(s). Good Samaritan laws are specifically intended to protect the well meaning person in these cases, even if that well meaning person actually does more harm than good.
Obviously, the claim of trying to deliberately "finish him off" would only apply in a shooting situation, not a car crash. But then your attorney could point out that you still had bullets in your gun and could have easily "finished him off" much simpler if that was your intent. I'm assuming here that you didn't empty your gun and still have a live BG on your hands. That would be a bit unlikely unless you are using a .22 or you are a really bad shot.
Your attorney can make quite a few arguments, but it won't change the fact that to most reasonable people, you have a vested interest.
In a car accident (MVA), you may have "heart interest", i.e., you genuinely want to help these people survive a traumatic event, but it will not financially, or in any other specific manner, benefit or cost you in any way.
Regardless of what you, or your attorney, says, there is quite a bit of truth in, "one story, end of story." IOW, my chances of life continuing outside of prison, crushing legal debt, and years in court, dramatically increase if the person I shot isn't showing up in a wheelchair, with his distressed momma, Awwntie, and brood of offspring, wailing about what a burden it is that this fine upstanding church-going, recovering addict, Daddy is no longer able to work part time at Dollar Tree.
Not meaning to drag this soooo far OT, but this is a serious topic, and I'm both licensed and have several years of pre-hospital experience in a fairly violent metropolitan environment.
Regarding the idea that "pain and suffering" are a result of the actions of the person you shot, that's very iffy. Legally, if you did not "stop the threat"(i.e. Tommy isn't dead), either your training and competence were inadequate, or
you felt the threat was stopped. It's either-or. The option of stating that you were, "so rattled that my aim was off," is the poorest of a set of bad options, regarding what to say.
I carry a pretty complete kit, but it's very much a limited use piece of gear. I carry a larger, multi-use bag to church, or select social gatherings.