It would have zero effect. There are lots of ads already against all kinds of violence, particularly in places like Chicago and Detroit. The problem is too complicated for public service announcements. A culture of violence has taken hold and I don't know how we stop it short of martial law in a lot of neighborhoods. An expanded ATF maybe? Aggressive enforcement of existing gun laws might help, but with so many guns on the street, the cat's out of the bag.
The Will Smith story is sad. But not unexpected. A rich guy with a gun was hit by a jerk with a gun. Someone dies. Story at 11. Can't anyone just exchange registration and insurance? Granted, if neither had a weapon, they are both alive tonight.
25 years ago if there was a traffic accident, how many people drew guns. We'll never be certain, but road rage and shootings post accident were still unusual in most places. It's more than just too many guns.
A callousness about life in general is now pervasive. Suicides are near record levels in both the U.S. (and Britian). Showing the victims dead bodies might help. I know it sounds terrible. But having highschoolers visit a morgue before graduating could scare people straight. "This is where you end up, like a piece of meat. Stop shooting each other." Desperate move? Yes. Could save one life.