Why did Nachash come now against Yavesh Gil'ad?
Malbim: He was jealous that Yisrael appointed a king, and wanted to fight them. He chose Yavesh Gil'ad, for presumably, the residents were from Binyamin, Sha'ul's tribe. After Pilegesh b'Giv'ah, 400 remaining Bnei Binyamin married the girls from there, after all the males and wives were killed, and the girls inherited their fathers' inheritance. 1
Perhaps he confronted Yavesh Gil'ad, for he knew that Bnei Yisrael killed almost everyone there. He thought that they are not so apt to come to help them, especially according to those who explain that Pilegesh b'Giv'ah was after Shimshon died, only about 50 years before this. (PF)
Why did he not simply confront a city of Binyamin? It is a Chidush that Nachash knew so much about the men from Binyamin, especially according to Seder Olam, that Pilegesh b'Giv'ah was shortly after Yehoshua died, almost 400 years ago! Also, Bnei Binyamin owned much more in Gevul Binyamin. Presumably, most girls inherited at most (and often less than) what her father's father owned. The 600 Bnei Binyamin who survived Pilegesh b'Giv'ah inherited the entire portion of Binyamin, which was given to 45,600 men; on average, each man received over 75 portions!
What kind of Bris did they want to make with Nachash?
Metzudas David: That you will not kill us.
Malbim: It was a Bris of love and Shalom.
