Now to the extra-question
/scoreboard players enable @a killall
here is the book:
/give @a written_book 1 0 {pages:["[\"\",{\"text\":\"Kill all entities\",\"color\":\"dark_gray\",\"clickEvent\":{\"action\":\"run_command\",\"value\":\"/trigger killall set 1\"},\"hoverEvent\":{\"action\":\"show_text\",\"value\":{\"text\":\"\",\"extra\":[{\"text\":\"Kill all entities in a radius of 20\"}]}}}]"],title:"Kill All",author:UncleThaodan}
Constantly running commandblocks:
/execute @a[score_killall_min=1] ~ ~ ~ kill @e[type=!Player,r=20]
/execute @a[score_killall_min=1] ~ ~ ~ clear @p written_book -1 1 {display:{Name:"One Shot"}}
/scoreboard players set @a killall 0
*Haven't tried it yet, but it should work*