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

Let me give you an example: 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?
The answer to this example can be: What's the meaning behind the phrase "The blood stays on the blade"?
Here is 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

OK. solve this:
In Let the Right One In (orig.: Låt den rätte komma in), after Oskar found out about Eli's true nature and let her into his apartment he asks her:   Oskar: Who are you?    Eli: I'm like you.    Oskar: What do you mean?    Eli: "What are you staring at? Well? Are you looking at me? So scream! Squeal!", those were the first I heard you say.    Oskar: I don't kill people.    Eli: But you'd like to, if you could...to get revenge. Right?    Oskar: Yes.    Eli: I do it because I have to. Be me, for a little while. Please... be me, for a little while.   But I don't understand what she means with her last words. What does she want to say with "Be me, for a little while"?
Answer:
What does Eli mean with the words "Be me, for a little while"?