Stepper Kill Switch

Other Hardware Motors, Mechanics, Power and CNC
8 
void loop() {
 
  digitalWrite(stepPin, HIGH);
  delayMicroseconds(100);         
  digitalWrite(stepPin, LOW);
  delayMicroseconds(100);
...

if (digitalRead(buttonPin) == HIGH) {         
      digitalWrite(stepPin, LOW);
}

You "disable" stepPin then immediately set it high again. Try

void loop() {
 
if (digitalRead(buttonPin) == HIGH) {         
      digitalWrite(stepPin, LOW);
} else {
  digitalWrite(stepPin, HIGH);
  delayMicroseconds(100);         
  digitalWrite(stepPin, LOW);
  delayMicroseconds(100);
}

Note also the use of the code tags (the # button) to make the code easier to read.

Rob