I know if I shoot an unarmed teenager that's 30 yards away the whole "HE WAS COMING RIGHT FOR ME" defense isn't going to fly. I'd be handcuffed and brought in immediately, and my ass would be sitting in jail while they did an investigation. The case wouldn't even go to a grand jury, the prosecutor would skip that and go straight to trial and I'd probably be found guilty.
Different set of rules for cops and normal citizens in this country.
That depends on the circumstances. If the teenager had already assaulted you, and was coming back at you, then you would probably have a pretty decent case for self defense. Normal citizens have gotten off with less of a defense.