In, "There Will Be Blood", what was the motivation to kill

Plainview was a very vindictive person. In his mind he felt extremely betrayed. He had put what little familial love he still had (after he banished his son) on his "brother". So when the brother turned out to be an impostor, Plainview felt his trust (something he doesn't give lightly) had been breached, and his violent nature took over. Notice how he brought back his son afterward, as he was the only family he had left.

I think Dan killed Henry because he realized that his son (adopted son) was trying to kill Henry that night when he set their room on fire because he knew that Henry was lying (he put oil around Henry's bed and then set it on fire). As a result Dan abandoned his son.

Daniel was not only deceived but then he sees that this person is completely different than him when he is asking him for money to continue on with the prostitute or whatever he is doing as he is laughing. Henry might not take enough from him to justify his killing before he is killed, but Henry certainly was looking to take a lot from Daniel via pipeline money.