The question "what killed the cat" is a common idiom that means "what caused a problem or failure." The phrase "curiosity killed the cat" is often used to warn of the dangers of unnecessary investigation or experimentation. The original form of the proverb, now rarely used, was "care killed the cat". The phrase "curiosity killed the cat" is only half the sentence, and the full sentence is much more uplifting. It ends with "but satisfaction brought it back". The second part of the saying shows that the risk might be worth it, and the cat gets to live. Therefore, the cat was not killed, and the phrase is used to warn people of the dangers of being too curious or nosy.