Act Two of "Macbeth" contains the famous "dagger” soliloquy. Why does Macbeth say the things he does in his private speech?
He is confiding in himself, trying to figure out what the best course of action is, and trying to figure out if killing Duncan is the best thing to do.
Normally, soliloquies are supposed to be a characters train of thought shown to the audience. That is exactly what Macbeth is doing in this speech. It is trying to show us that he is conflicted, that is he worried, that he is trying to convince himself that this is the right thing to do.
If you still need more help, I recommend this translation. It's not too true to the original, but it might help you understand what he is trying to say in modern terms.