I have a robot which runs on 24v with 4 motors each rated at < 2300mA. I would appreciate if someone can share a circuit or something that hopefully implements a kill switch using a relay switch that can be controlled via RC or MCU.
Do you have a spare channel on the radio system?
If it's for safety, the best way is the R/C equivalent of a Dead-Mans-Handle (as used in old trains).
If the signal disappears, it halt the vehicle immediately.
If you have to send a signal to stop it, what if something interferes?
How it reengages (signal reappearing or a reset button on the 'bot) can be left to personal preferences.
If you have a spare channel, you could set it to send eg. 2ms pulses constantly and a small microcontroller on the vehicle (dedicated to this single task) can decode the signal and unless there's a constant 2ms pulse train, it stops energizing a relay. This relay should be wired to feed the power through its contacts when the relay is engaged and will then cut power if the signal isn't received within eg. 50ms (which allows a single pulse to go missing).
You can stop it manually as well, by changing the 2ms to eg. 1ms (by moving a stick from one extreme to the other, or you could use buttons if your equipment has got any.