Anything that arrives on the serial port pins, because the PC sent stuff via the USB cable, or because the Arduino wrote to the pins using Serial, is broadcast by the XBee connected to the same two pins.
The Arduino does not need to do anything to make that happen. It does not even have to read the serial data.
If the XBee is connected to two other pins (hardware or software serial pins), then the Arduino does need to read the data, using Serial.available() and Serial.read(), and it needs to send the data to the other pins, using the appropriate serial object.
The receiving Arduino appears to expect characters. Why you are using the binary function to send character data is mystery.
Nothing that the Arduino connected to the PC is doing should cause the PC to crash.
void loop() {
performCommand();
}
Why?