Radak: It is one of the gates of Yerushalayim. It is called Sha'ar Binyamin because it was in Binyamin's portion, or because outside the gate was the road to Eretz Binyamin.

Rashi: He is appointed to guard that no one leave the city (Radak - via that gate) to make Shalom with the Bavliyim.

Rashi: It was Chananyah ben Azur. This teaches that anyone who flatters a Rasha, in the end he falls to him, his son or his grandson. Yirmeyah answered "Amen

Radak, Targum Yonasan: You join with them and heed them. This is like "umi'Menasheh Naflu Al David" (Divrei ha'Yamim I, 12:20). Malbim - because they were still in Yehudah, they suspected that he wants to flee to them to be with them.