GTA IV 10 points for best answer?

May be...

In execution situations, after u aim what do u press to let someone live

you just walk away

If you're in a position like that in GTA IV don't press anything. Dont press the shoot button. Just walk away from them. They will still live, or just aim at something else.

you don't have to press anything, all you need to do is walk away from them and they'll live

walk away

walk away from them