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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: I'm a bit confused about a scene in How To Train Your Dragon. When the Vikings reach the nest, they open holes and the dragons are flying away. Why didn't they do that in the first place and wait for the humans? I mean, they had input/output ability from the nest (they could fly to the village to steal some food). Could it be that the queen kept some dragons as "prisoners" and only a subset of the dragons were going to the village, so that they are doomed to get back in the nest, so that they didn't lose their friends (just a thought, not a clue of this in the movie)? I was wondering if someone could explain this to me or if it really is a plot hole in the story.
Solution:
Why didn't the dragons leave their lair earlier?