Question

why can small mass such as a bullet can kill a person, when fired from a gun

Answer 1

due to high velocity.. .....

Answer 2

It is shot at a high speed, it packs a large kinetic energy and also force.

Now, the surface area of contact is very little for the bullet , and it is also pointed , pressure is inversely proportional to the area contact. Thus pressure is very high for the bullet and due to high pressure it can go deep into a persons body.

This thus injured his organs or his blood vessels thus causing danger to his life.