Although the phrase “take no prisoners” sounds merciful on its surface, it usually refers to taking an overly aggressive stance in a particular situation. It usually implies that someone lacks mercy, but its broad range of applications does not always translate to being merciless. The likely origin of the phrase is as a command in combat, which implies that opposing forces would be killed rather than spared and taken into custody.
To “take no prisoners” would not be a common military command in modern times. Killing wounded soldiers or soldiers who have surrendered are considered to be international war crimes. Fighting so that there are none left to surrender or so that people are killed rather than wounded, however, would be a style of warfare that is not technically illegal.
A similar expression is the British military term of “giving no quarter.” This also can be translated as being aggressive in combat. The term "quarter" essentially refers to living arrangements for prisoners. Thus, giving no quarter can be construed that have the same meaning as taking no prisoners.
In modern usage, one might see “take no prisoners” applied to a person’s approach to politics or athletics, styles of written and visual arts or speaking abilities. It also might reflect on a person’s parenting abilities or other styles of living. For example, the phrase could be used for a book that includes extremely graphic violence and is written in a style that does not spare the reader any details. A parent who punishes all children for the misbehavior of one child or punishes children for minor transgressions might be adopting a “take no prisoners” approach. A politician who delivers an aggressive and vindictive speech against an opposing party might be considered to have this type of attitude.
This style often implies viewing the world in limited terms because there is no dimension in which mercy can be dealt. Instead, there is merely forward, active aggression from which no one is spared. Mitigating circumstances of injury or surrender — or their equivalents in non-military settings — do not exist in this mind-set.
Aggressive or decisive action is not always negative, however. A person who relentlessly pursues a goal and is not distracted by difficulties along the way may be quite successful. A student who combats health issues or lack of money and is able to get a degree may have done so in an aggressive way, but with positive results. Sometimes an objective requires unilateral focus in order to be achieved.