The phrase "killer instinct" refers to a competitive spirit or an aggressive drive to succeed in a particular field or activity. Some synonyms that can be used to describe someone with this quality include ferocity, tenacity, ambition, competitiveness, and ruthlessness. Other words that may be used to convey the same meaning include determination, fortitude, perseverance, relentlessness, and grit. These words all suggest a willingness to push oneself to the limit and to do whatever it takes to achieve success. Having a "killer instinct" can be a valuable trait in many areas of life, including sports, business, and politics.