Question

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

Answer 1

It kills a person instead of having small weight because of its force. The force exerted by the bullet is very high

Answer 2

It is so because even if the mass of the bullet is small, it moves out of the gun with a very high velocity due to which the momentum produced is high (p = mv). This high momentum of the bullet kills a person.