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Given scene/plot descriptions from movies or TV shows, and given a specific question about the scene, summarize the question into a title

At the beginning of the movie Gangs of New York, Priest Vallon tells his son not to wipe the blood of the blade after he's cut himself shaving, saying "The blood stays on the blade". I've watched the movie multiple times, but can never find what the meaning behind that phrase is. Anyone able to shed some light on this?
Solution: What's the meaning behind the phrase "The blood stays on the blade"?
Why? A person gives a detailed description about a scene from the movie Gangs of New York and asks the meaning of a said phase in the scene. The question is summarized as What's the meaning behind the phrase "The blood stays on the blade"? for easy indexing the next time someone searches for a similar phrase

New input: Near the end of The Raid (2011), the lieutenant Wahyu and Dagu reach the boss, then Wahyu kills Dagu. But he is not on the boss' side (and later kills the boss anyway) so why kill his comrade?
Solution:
Why does Wahyu kill Dagu?