What is "[Za'ak] v'Heilil"?
Rashi: It is an expression of Yelalah (wailing). Radak ? people will not rejoice ? "Oh Nasis" (4).
Rashi (18): Do not rejoice over this (that Yehudah despises idolatry), for the matter is already tested in front of Me; they will not properly repent.
Malbim: He should scream due to the sharp side of the sword, and lament due to the dull side that [Melech Bavel] held. It was the means for him to conquer them! I.e. the sword of Yisraelim killing each other, which is the reason for all evil.
What was "b'Ami"?
Rashi: That sword. It was also "b'Chol Nesi'ei Yisrael" and the officers.
Malbim: The primary sword was in My nation - Yisraelim fought and killed each other.
What does the verse teach about "b'Chol Nesi'ei Yisrael"?
Rashi: That sword will be also against them, and the officers.
Radak: Yoshiyah and Yehoyakim died via the sword, and Tzidkiyah's sons were slaughtered in front of his eyes. Those who were exiled, it was due to the sword that passed over them.
Malbim: The sword (war) was also between the king and his officers, and the nation.
What are "Megurei El Cherev"?
Rashi, Targum Yonasan: Gatherings of nations coming to the sword were around My nation. Megurei is like "Oger ba'Kayitz Ben Maskil" (Mishlei 10:5).
Rashi citing Menachem: It is an expression of fear, like "va'Yagar Mo'av (Bamidbar 22:3). Radak ? they feared the sword before it came ? "Cherev Yeresem v'Cherev Avi Aleichem" (11:8). "Megurei" is Samuch to "El", just like we find words Samuch to 'Alav' and 'Es' ? "v'Holechei Al Derech Sicho" (Shoftim 5:10), "u'Mesos Es Retzin" (Yeshayah 8:6).
Malbim: They feared the inner sword (within Yisrael), which killed more than the enemy did via the siege.
Why does it say "Lachen"?
Radak: Because the sword was against My nation and all Nesi'ei Yisrael?
What do we learn from "Sefok El Yarech"?
Rashi: He slaps his thighs to lament. Radak ? "El Yarech" is like Al Yarech.
Sha'arei Teshuvah (1:18): After I informed people of my sins, I mourned via acts that people see, e.g. slappingmy thigh.
Malbim: He claps his hand (laments) due to the sword on the thigh; people hang the sword there ? "Chagor Charbecha Yarech Gibor" (Tehilim 45:4). This is the sword of Yisrael against each other.