1) THE ANGEL "LAILAH"
QUESTION: The Gemara quotes Rebbi Yochanan who teaches that when Avraham Avinu fought the war against the four kings, he was assisted by the Mal'ach named "Lailah." Rebbi Yitzchak Nafcha says that Avraham was assisted by the stars.
The Gemara in Nidah (30a) teaches that when a child is conceived, the Mal'ach named "Lailah" takes the newly conceived embryo and brings it to the Heavenly court that decrees upon it all of the qualities that the person will have, except for the trait of Yir'as Shamayim.
Why was this Mal'ach in particular involved in fighting Avraham's war?
ANSWER: The MAHARSHA explains that Avraham Avinu did not have the natural power to overcome the nations in war, and he certainly was not able to conquer the most powerful kings in the world. As Avraham Avinu said, "I should have been dust" under the feet of the kings (RASHI to Bereishis 18:27). Avraham Avinu persevered only because he and his nation are above the natural order of the world, as the Gemara in Shabbos (156a) teaches about Avraham Avinu when it says that he bore children even though he was physically not supposed to have children. Through Tefilah and Zechus, one is able to alter his fate.
This is the allusion of the Gemara here when it says that "Lailah" fought on behalf of Avraham Avinu. The same Mal'ach that gave him his destiny when he was conceived now came to change his destiny.
Rebbi Yitzchak Nafcha makes a similar point. The Mazal of a person is governed by the positions of the stars at the time of his birth (as the Gemara in Shabbos says). The same stars that gave Avraham Avinu his original Mazal now came to change his Mazal so that he would be able to survive.
This might be why, immediately after the Torah relates the incident of the war against the kings, it relates that Avraham Avinu had a prophetic vision in which he asked Hash-m for children (Bereishis 15:1). Hash-m brought him outside and showed him the stars and told him that he would have as many children as there are stars.
The Gemara in Yevamos (64b) says that Avraham and Sarah were born without the ability to have children. The Gemara in Shabbos (156a) explains that when Hash-m answered Avraham Avinu by telling him to look up at the stars, he was telling him that he is not going to be governed by his Mazal. Although his Mazal was to remain barren, Hash-m was changing his Mazal by moving around the stars so that he could have children. It was only after Avraham Avinu fought against the kings, at which time he merited to rise above his Mazal through his faith in Hash-m in making him victorious, that he merited to rise above his Mazal with regard to bearing children as well.
2) LETTING THE DESCENDANTS OF "RESHA'IM" JOIN THE JEWISH PEOPLE
QUESTION: The Gemara points out that many of the greatest enemies of the Jewish people eventually joined the Jewish nation in some way. Nevuzaradan, after he killed nearly a million Jews and destroyed the Beis ha'Mikdash, converted and became a Ger Tzedek. The grandchildren of Sisera became Gerim, as did the descendants of Sancheriv and Haman. However, the Mal'achim did not allow Nevuchadnetzar to have descendants who would be Jewish. The Mal'achim argued to Hash-m, "How can we accept a person who destroyed Your Beis ha'Mikdash to come beneath the wings of the Shechinah?"
Why did the Mal'achim not protest when the other Resha'im or their descendants joined the Jewish nation?
(a) REBBI TZADOK (in MACHSHAVOS CHARUTZ, p. 178, and DOVER TZEDEK, p. 174) explains that the destruction of the Beis ha'Mikdash and the removal of the dwelling of the Shechinah in this world can be brought about only by a person who has absolutely no good or positive elements left in him. The reason why grandchildren of Resha'im convert is that they bring the positive traits hidden in their grandparents into the Jewish people. Since Nevuchadnetzar had no positive traits, it was not necessary for his descendants to join the Jewish people.
Why, though, was Nevuzaradan accepted as a Ger, if it was he who personally destroyed the Beis ha'Mikdash and killed so many Jews? It seems that if he was able to regret his evil deeds during his lifetime, then that shows that he had some element of good inside of him.
(b) RAV YAKOV KAMINETZKY zt'l (in EMES L'YAKOV, Bereishis 27:40) explains that the reason why the other Resha'im merited to have their descendants join the Jewish people is that they brought about a Kidush Hash-m, albeit unintentionally. Through the actions of Haman, the Jewish nation repented for their sins and the miracle of Purim occurred. Similarly, the miraculous destruction of the armies of Sisera and Sancheriv brought about a great Kidush Hash-m.
However, Nevuchadnetzar succeeded in his evil plans, and he was not miraculously defeated. Therefore, he did not merit to have his descendants join the Jewish nation. (See also KEDUSHAS LEVI, Kelalos ha'Nisim, and CHOCHMAH U'MUSAR 2:345, who offer similar explanations.)
Rav Yakov Kaminetzky adds that when the Gemara says that Hash-m wanted to bring the descendants of Nevuchadnetzar into the Jewish nation as well, it means that He wanted to burn Nevuchadnetzar with the other kings who helped him throw Chananyah, Misha'el, and Azaryah into the furnace (see Sanhedrin 92b). If Nevuchadnetzar would have been killed by the miracle that saved Chananyah, Misha'el, and Azaryah before the eyes of the nations of the world, he would have been part of a Kidush Hash-m and would have merited that his children join the Jewish people. The Mal'achim protested, however, and Nevuchadnetzar's life was spared.