Did you set the mechanical linkage to allow it to bottom out and kill the engine before you started playing with the digital trims? I always set my throttle connections with 100% stop-to-stop mechanical throw before I start to play with the digital trims and sub-trims.

I use throttle-down and kill on two seperate switches on my Futaba 9-C. Don't have any experience with a DX-7 or Spektrum.

Just an outside chance - this is not a gasoiline engine is it? They need a seperate circuit to ground the spark and not just a low throtte kill.