A punch can actually kill someone.
A decent punch to the gut can puncture the blood vessel that will cause internal bleeding. Also the neck being a dangerous place to hit, if hard enough it can break the blood vessel as well.
Yes and it can also kill someone.
A heart thrust punch can create such abrupt trauma to the sternum that it can cause the heart to bruise and even stop.
Kinetic Linking done properly (taught in most Martial Arts) can have devastating effects.