Daniel killed his own step-brother because he felt betrayed by him, he disowned his own son because he felt threatened by his business ventures, and finally at the end of the movie, Daniel kills Eli as an act of taking revenge. The whole of the last scene of the movie essentially revolves around vengeance.

What did Daniel Plainview whisper to Eli?

Right after the baptism, religious music plays and we see Daniel facing away from camera say something to Eli, who then looks horrified. I'm guessing either "there will be blood" or something along the lines of "I'm going to eat you", as he later yells "I told you I would eat you!"

Why does Daniel Plainview put a napkin over his head?

I forgot how much I loved the restaurant scene in 'There Will Be Blood', when Daniel Plainview (Daniel Day-Lewis) didn't want his deaf son HW (Dillon Freasier) to read his lips, so he covered his face with a napkin….