What was Reuven's motive in stopping his brothers from killing Yosef?
Seforno: He prevented them from acting (killing him) suddenly. 1 This leads to acts that cannot be fixed.
Ramban (on Pasuk 26): Refer to 37:26:1:3.
Ohr ha'Chayim: People have free choice, so they can kill one who is not Chayav Misah. Animals would not kill one who is not Chayav Misah. Reuven saved him from their hands. Since they have Bechirah, killing him would not prove that his dreams were false. 2
Seforno: Also a Tzadik does so sometimes, like Reuven himself did with Bilhah, about which [his father would rebuke him (49:4) for being "hasty like water". He was currently doing Teshuvah, as the commentaries explain, based on Bereishis Rabah (84:19). (EC)
Ohr ha'Chayim: Reuven was sure that animals would not kill one of Yakov's children. He intended to return Yosef home before he would starve.
What reward did he receive for this?
Makos, 10a: That is why in the Parshah of Arei Miklat, Reuven is mentioned first. 1
Why does it say va'Yatzilehu, and twice "va'Yomer"?
Malbim: First, Reuven tried to totally save him (from any punishment). They did not consent. Then he said that they should not kill him. Also this they rejected. Finally, he said that they should not overtly kill him, rather, through Gerama (causation).
Ha'Emek Davar: Only the second time, it says "Lahem". The first time, he screamed to himself - we cannot kill him! Then he spoke gently but forcefully, and explained why they must consent.