De inimico non loquaris, sed cogites.
Do not wish ill for your enemy, plan for it.
I saw this quote recently in either a Vince Flynn, Brad Thor, or Lee Child book (I can't recall which author/book because this summer I've read all of their books - which are all great!). The quote really fits the Mitch Rapp / Scot Harvath / Jack Reacher persona. I've spent a few minutes on a Google search but haven't been able to find the origin of the quote.
It's quickly become one of my favorite quotes - It just sums up the world today.