I have the 14sga transmitter. I have a kill switch on ch 7 sw se. I would like to set
failsafe to activate the kill switch if a problem occurs. How can I do that? I have it
set now to go low idle.

Which receiver are you using? That's important. With an 8-channel or more receiver, you just set each channel to the position you want in the failsafe function in the Linkage Menu of the transmitter.

Receiver is R7008sb

There you go. Set failsafe in the transmitter. See page 80 of the instruction manual.