I assume it's a reference to a James Mattis quote. > Be polite, be professional, but have a plan to kill everybody you meet. It refers to a general concept of

I assume it's a reference to a James Mattis quote. > Be polite, be professional, but have a plan to kill everybody you meet. It refers to a general concept of alertness and awareness about your surroundings, and how you can't tell who might be good or evil until they act, so just assume everyone could be malicious until proven otherwise (or, more practically, until you're out of their sphere of influence).