Tamar and Judah

In Genesis 38, Tamar is the daughter-in-law of Judah. She marries the oldest son, Er, who God struck dead for reasons unknown. The next son, Onan, was to marry her in order to give his brother offspring. He would not do so, and "spilled his seed on the ground" when he slept with her. This made God angry, and he struck Onan dead. The last son, Shelah, was next in line to marry Tamar, but Judah sent her away until he grew up, or so he said. Judah went into town with Shelah to shear his sheep, and Tamar went to see that he was grown and that he had not been married to her, so she dressed up where she would not be recognized. Judah saw her and mistook her for a prostitute, and slept with her. She took his forms of identification to know who the father of her child was if she conceived. When Tamar became pregnant, she showed the items and said they belonged to the father of her children. Judah recognized them and said "She is more righteous than I, since I wouldn't give her to my son Shelah" (Gen 38:26). She gave birth to twin boys, one of which is an ancestor of Jesus.

Why is the reason for Er's death unknown? All that we have to go on is that he was "wicked in the Lord's sight" (Gen 38:7). What does this mean? What could he have done to deserve this punishment? Maybe he could have killed a man. We do not know, because the Bible is cryptic.

So how did Tamar go about reproducing with Judah? Was it unintended and just happened because she was looking to see if Shelah had grown up? Or did she plan this out in advance? My understanding is that it was the latter, because she took the time to change out of her widow's clothes.