/Kill is simple as ever. /Kill is basically like this: you do /kill (player) then it kills that player. But if you do /kill @r means you kill a random person here: @r = random @e = all entities @a = all players @p = nearest player and @s = yourself. You have to be on OP to do it though. OP = Admin = Operator.
To be admin, you need permission, or you have to create that world. OP people can turn people on visitor, Admin, and member. But if you turn somebody on admin then they turn the person that created the world on member, the person that created the world can still turn themselves back on Admin.