So you wanted, for example, to make a machine to kill the players (Specific, that's the point)? Well.
/scoreboard objectives add kill dummy - Will add a dummy objective named 'kill'.
Then place a command block with the command 'kill @e[score_kill_min=1]' or for 1.9, 'kill @e[tag=kill]' - It's clear. Anyone who has this objective set to 1 or higher OR has tag 'kill', will get a kill.
The fact, if you use it on multiplayer, it will require you being OPerator on it (since when you could /scoreboard while being a normal player?).
But for Single-Player: Perfect!
You /scoreboard players set <name or selector, for example 'Kneesnap' or '@e[type=Spider]> kill 1.
OR for 1.9: /scoreboard players tag <same as objectives> add kill
Then you make a repeater pointing off the first command block. And place a second CMD-block. You type the command 'scoreboard players set @e[score_kill_min=1] kill 0' or 'scoreboard players tag @e[tag=kill] remove kill'.
And place a button on the first command block.
Now you have a configurable machine that can be used only by OPs or if Cheats Enabled in Single-Player world.
Note for AbsentFuture: If that was already done, provide example.