Why dose the starter engage when the key is off and the clutch is depressed?

Yikes, sounds like your ignition switch is stuck in the "crank" position. If so, the only thing stopping it from turning over all the time is the clutch peddal switch.

The switch I'm talking about is in your steering column, at the back of your key cylinder. It is black and plastic with 2 plugs into it. When mine went bad, I wasn't able to start the vehicle, but in theory if that contact were to stick, it would cause your problem.