Why does it say "Gam Achecha..."
Malbim: Also the Kohanim in Yerushalayim...
How did they betray him?
Radak: They wanted to feed poison to him.
What is the meaning of "[Kar'u Acharecha] Malei"?
Rashi: A group of people [called against you]. Radak - to scare you and confound you if you will prophesize for them. Also "Melo Ro'im" (Yeshayah 31:4) is a gathering of shepherds against a lion.
Radak citing Rav Hai Gaon: "Malei" is opposite; it really means empty of everything. This is like a blind person is called Sagi Nahor (great light).
Radak citing his father: This is like "Asher Mela'o Libo" (Esther 7:5). They say, not in front of you, that you have no fear in the world. They tell you, "do not prophesize in the name of Hashem, and we will not kill you" (11:21), and you continue to prophesize, without fear of death!
Malbim: Also they called against you to kill you.
Why should he not believe them?
Radak: If they bring you close and speak nicely to you, do not believe in them, for they speak Sheker. They want you to be close to them in their houses to eat and drink with them, and they will give to you poison to eat or drink.
Radak citing Targum Yonasan: You Yirmeyah request mercy for them (Anshei Yerushalayim. Know that) just like your brothers and father's house lied to you, also they spoke bad about you; do not believe when they say good things.
Malbim: What they say with their mouth is not sincere. Therefore, also in Yerushalayim your rebuke will not help.
