Originally Posted by Raleighcopter

The Rcexl Kill switch switches the ground connection to the battery. When you install an Arduino, it provides another ground path via the rpm ground. As a result, the kill switch is bypassed through the Arduino. An opto-isolator would solve the issue but removing the ground from the rpm plug works too as the kill switch provides the needed ground reference when the ignition is enabled. Of course, when the ignition is disabled, the RPM is zero.