What is the meaning of the last scene in the movie There Will Be Blood?

Revenge.

While the whole film deals with Daniels inhumanity in different ways, the 'tit for tat' relationship between him and Eli is clearly episodic, with its climax in the fantastically theatrical final scene in the private bowling alley. Years after suffering his final humiliation at the hands of Eli (the baptism), who by the end of the film may have become the hypocrite Daniel always thought that he was (drinking, commercially driven and bathing in the vanity of his broadcasting), Daniel sees that Eli has come to visit, desperate for money.

After making the suggestion to Daniel that they buy a piece of as yet undeveloped land in Little Boston (the land belonging to Bandy which Daniel tried to buy in the main body of the film), Eli confesses that he has succumbed to various vices and is desperate for money. Daniel agrees to buy the land providing Eli embarrasses himself in a similar way in which he forced Daniel to back in the church at Little Boston, making him renounce his faith and declare that he is a 'false prophet'.

After ensuring that Eli has well and truly humiliated himself, Daniel then proceeds to completely destroy his soul by following the humiliation with the statement that the Bandy tract is worthless, having been drained by Daniels adjacent land ("Milkshakes, Drrrrrrrainage" etc),taunts Eli with the suggestion that his brother, Paul now has commercial success (possibly a lie?) then, upon seeing that Eli has lost all hope in this desperate venture, completes his life time goal to one-up Eli by destroying his body (by using the bowling ball to stove his head in).

After which he is of course, 'finished'.