Without knowing the reason your co2 went out of control, I can't really give you a good answer. Here are a couple ideas.
Chances are your needle valve was opened up accidentally, either this or it was set too high to begin with and the regulator was opened up further. Preventing either of these is difficult using the Milwaukee setup. Once the needle valve and regulator positions are set, there is no way to remove the turn knob of these devices. There are regulators and needle valves on the market that either A)require a screw driver to adjust the setpoint or B)have removeable knobs. If this is a very strong concern of yours, you can sell your regulator on ebay and buy one of these. Pacific Coast regulators are like this, at least for the regulator pressure(not the needle valve).
My other guess is that you had the co2 set too high to begin with, but it wasn't high enough to kill the fish before lights out and when you woke up in the morning they were dead. The way to prevent this is to have your co2 on a timer, shutting off at lights out and coming on an hour before lights on...this way when the co2 is on, you can see what's happening. This isn't perfect, but it helps.
And finally, you can also use a pH controller, that will open and close your solenoid valve depending on a preset condition of your tank. This way, even if the needle valve is open too far, once your co2 hits a certain setpoint, the solenoid will close and not release any more co2.
This happened last Saturday. I've had the cylinder running with just the plants since then. The pressure is reading at just a shade below 100. It's at about 90-95 psi.