Do you just want something to die. Well look no further for I have the command for you. If you type

/kill

that kills you but if you put

/kill username

that kills whoever has that name. If you put

/kill @e[type=entity]

that kills all the things in your world that match that entity.

/kill @e

kills everything

/kill @a

kills all the players on the world

/kill @e[r=4]

Kills anything in a 4 block radius of you. You can change it to any number.

/kill @e[name=name]

kills mobs that you naked just turn the 2nd name to the name of the mob.

Have fun killing!