Sort of like the Alien's acid blood.
"It's got the perfect defense mechanism, you don't dare kill it."
I remember the exact second I killed the alien and learned it didn't bleed acid. I was 12 years old. It was the most liberating moment of my life--not because I realized there was no god, even as a kid I knew I had no way of knowing that for sure. It was because I realized if that was what god needed to keep his believers in line, he was an evil butcher who did not deserve to be worshipped.