Oh I missed that point. Asking for mercy is ok. He is also a Human Being but should not be pardoned. The easiest way to kill him is to cut his head as scientifically you will have a very little pain if you die like that. Though it looks horrible to see blood shed but the person who is dying will have no pain as soon his veins disconnected from each other.

To kill someone to protect other innocent human beings or your own self in defence will not make you a killer. In my opinion you will be a hero.