1)
Why did he say "Lo Chen ha'Davar"?
1.
Malbim: You are not "Shelumei Emunei Yisrael." The head rebel is among you!
2)
Why does it say "Nasa Yado ba'Melech b'David"?
1.
Rashi citing Midrash Koheles 9:18: He is Chayav Misah for rebellion against the king, even if it were not David. He is Chayav Misah for rebellion against David, even if he were not king, and all the more so for David ha'Melech!
2.
Radak: Even though Avshalom rebelled, David is the king, and one who rebels against him is Chayav Misah and will be killed, like Avshalom was killed.
3.
Malbim: The king is a Tzadik who rules with Yir'as Shamayim, so [if you have amidst you one who rebels against him, you are not "Shelumei Emunei Yisrael"!
