The Glock 17 is difficult to conceal, and impossible to conceal well with a T-shirt. I carry a Glock 19, which is more compact, and is less difficult to conceal, but still rough with a T-shirt.
As for your scenario, would you do something like that, and killed the guy, you would be charged with murder. Even if you miss, you're going to be charged with something, and you'll probably never be able to own a firearm again. You are only permitted to use deadly force if you are in immediate danger of great bodily injury or harm. Once the danger drives or walks off, it's a matter for the police.
I would highly encourage you not to carry a firearm until you get some training on the legal aspects of using deadly force in self-defense, and the legal issues involved with carrying a gun. At the very least, you should buy and read Massad Ayoob's classic on the matter "
In The Gravest Extreme", but getting training is well worth the money. If you're unsure of where to go, feel free to post another post asking, and I'm sure people will suggest someone near to you that will help with the training.