If you have a negligent discharge with a modified trigger, that possibly / probably will be an issue. If you intentionally pulled the trigger and your defense is justifiable homicide, I have to figure it's going to be less of an issue how light your trigger was. If someone runs someone over in their car by accident and they were drunk, I'm sure it's going to be a problem. If someone was drinking and states that they ran someone over on purpose, a drunk driving charge is probably not what they have to worry about.
In an intentional self defense shooting, the whole light trigger thing probably can really only be spun into the idea of your frame of mind since you were intent on pulling the trigger regardless of if it was 3 pounds or 13 pounds, and that's a weak argument of a desperate prosecutor IMO. If this is their entire case -- if they are trying to convict you on the basis of you must be blood-thirsty or incompetent because you have a 4# instead of a 7# trigger -- then you're probably in good shape with any but the most inept defense lawyers. We aren't talking about the fact that you were wearing a t-shirt at the time of the shooting that said "Kill 'em all and let God sort 'em out" on the front. We are talking about an otherwise safe, but slightly modified, handgun.
Most of these "target guns" are labeled as such because of (1) guns with lighter triggers tend to be using lighter springs which makes them less reliable in extreme cases and/or (2) there is a fear of a negligent discharge in the heat of an adrenaline-filled self defense encounter. If you have tested your pistol to be 100% reliable with your chosen ammo and if we are specifically talking about cases where the weapon was intentionally fired in SD, neither of these distinctions between a "target" gun and a "defense" gun matters.
So much of what get tossed around as commonly accepted internet gun wisdom has its roots in what some "expert" or another said... then it gets repeated and repeated and repeated until everyone basically takes it as gospel. Modify your gun and go to jail. Use reloaded ammo and go to jail. Put Punisher grips on your gun and go to jail. Use a .44 magnum and go to jail. Shoot someone from more than 10 yards away and go to jail. The fact is that we need to SEE the number of people who got prosecuted due to these things vs the number of people who did these things and we either not charged or not successfully prosecuted. I have never seen these types of numbers. In fact, I've seen next to no actual cases where these so-called common knowledge no-nos landed someone in prison in a case where, had someone used a non-modified gun or shot from only 9 feet instead of 29 feet or whatever, they would have gotten off free and clear. Jenny McCarthy read a bogus "scientific study" about a link between vaccination and autism, decides to spread the word, and now we have cases of measles and other previously eradicated illnesses on the rise. Free internet advice is often worth less than you paid for it.
Here is how I see it. Like I mentioned before, you have the risk of running afoul with the law from the second you buy a gun. Carry one at your own risk. Use it at your own risk. Heaven forbid you feel the need to use it and you will set into motion a whole series of events and processes that will be less than enjoyable (not even including the psychological effects of using deadly force regardless of if it was justified or not). Your fate will largely be guided by the cops that show up, the bosses of those cops back at the station, the DA that reviews the facts of the case, the media and the public at large, the lawyer you might need to hire, the lawyer that the other person or person's family might hire, etc., etc. Weigh the risks and make your choice. Chances are, if some or all of those people named above don't like you or don't like guns or don't like citizens using deadly force, they will make your life very hard -- trigger or no trigger, modification or no modification, good shoot or not. If some or all of those people feel like you did the "right" thing in the eyes of the law or of God or of their own moral compass, they will not make your life very hard -- trigger or no trigger, modification or no modification. A good shoot is NOT always ruled a good shoot, and people with non-modified firearms in jurisdictions that have castle doctrines or "stand your ground" laws DO get prosecuted sometimes. Decide what matters most to you... having a pistol that might help you hit what you're aiming at or make it less likely you hit a bystander or whatever... or possibly giving a tiny bit less ammunition to a prosecutor that probably already was gunning for you regardless. I really REALLY want to see an otherwise justifiable homicide case where the sole factor in someone going to jail was because they put a much lighter mainspring into their j-frame.
The argument that you should just "train harder" to get around the drawbacks of whatever platform you're using is a straw man argument. It's hard to argue that someone isn't a "better" shooter with a better trigger, no matter how good you are with a worse one. What you're suggesting is that you should train harder until you consider yourself GOOD ENOUGH to adequately overcome a heavy, gritty trigger -- despite the fact that, even at that level of proficiency, you would be BETTER with that same gun if it didn't have that heavy, gritty trigger.
If you have a $3k Wilson Combat 1911 with a 3.5# trigger that shoots 1.5" groups straight from the factory, is that LESS of a liability than your Glock that had trigger work done to drop the trigger pull to 3.5# from the factory 6+ pounds? Is a 1911 more of a liability than a double action gun that you can't shoot nearly as well simply because you walk around with the hammer cocked all the time? Is a Beretta with a safety and a 12# trigger pull less of a liability than a SIG Elite SRT without a manual safety? I can't answer any of these questions for you. Just use some common sense and answer them for yourself in a way that makes you most comfortable.
Oh... and in case anyone is misunderstanding what I'm saying, I'm not advocating that you carry a modified gun or not. I, myself, happen to have a carry rotation that does not include any modified guns. However, I don't get all bent out of shape about doing so... it just so happens that I am happy with the way my guns perform as is. I DO have a concern about accidental / negligent discharge in a high stress situation with a gun that has an extremely light trigger... intentional discharge with those same guns? Not so much.