"Do not do good to an evil person, and evil will not reach you."

 There is a point in not doing a favor for someone who will use it against you. In the Midrash i brought a story about some one who came from Israel to Persia and saw a dead bird. Then came another bird with a leaf from a plant, and put it on the dead bird, and that bird came back to life. Then he went to that same plant and picked a leaf from it, and said to himself,
"This will come in handy." On the way, he saw the carcass of a dead lion. He put it on the lion, and it came back to life and ate  him. From this the Midrash learns: "Do not do good to an evil person, and evil will not reach you."