What is the meaning of "Im Lo ha'Sechok"?
Radak: This is a question 1 - did you not laugh at Yisrael? So you will be a mockery!
Malbim: You laughed at Yisrael, that some of them returned stealthily to their land after their exile, and you depicted them like a thief who tunnels into houses at night.
Presumably, the text of Radak should be 'b'Derech She'elah', like Abarvenel says; 'Hash'alah' is a printing mistake. (PF)
Why does it say "Im b'Ganavim Nimtza"?
Radak: Was Yisrael found among thieves, that you mock them like one mocks and exults over a thief [who is caught], like it says "Yari'u Aleimo ka'Ganav" (Iyov 30:5)?
What is the meaning of "Midei Devarecha Bo Tisnodad"?
Rashi #1: [Amidst discussing Yisrael,] you shook your head and mocked our Churban.
Rashi #2: Because you laughed over Yisrael's Churban, you will be exiled. Also Targum Yonasan says [similarly] because you spoke many words about [Yisrael], you will be dislocated.
Radak: Whenever you spoke about Yisrael's exile, you laughed, like one who delights in a matter and laughs and dances.
Malbim: Like you said that Yisrael returned stealthily to their land like a thief entering others' houses, so you will do!