take-no-prisoners mentality

Before I Say Good-Byeを読んでいます。
And of course there was more, Nell told herself, so say it straight. Adam was in some ways the antithesis of Mac. I know how Mac feels about me, she thought, but he would choke on the word “love.” I was hungry for someone to tell me instantly, passionately that I was loved.
But in other ways, Adam and Mac were very similar, and I liked that too. He didn’t so much have the take-no-prisoners mentality that Mac had, but he had the same moral stamina. Adam, like Mac, was very independent, having worked his way through college and graduate school.
"take-no-prisoners mentality" の意味が分かりません。"take-no-prisoners" を辞書で調べます。
・Merriam-Webster's Online Dictionary: having a fierce, relentless, or merciless character: take-no-prisoners politics
・Dictionary.com: wholeheartedly aggressive; zealous; gung-ho: a businessman with a take-no-prisoners attitude toward dealmaking.
何故この様な意味になるのか The Phrase Finderに次の説明がありました。
She doesn't spare anyone. If you take no prisoners, you kill all of your enemies. There are none left alive to be prisoners of war.
なるほど、皆殺しで捕虜は不要ですか。恐ろしい。