Why does it say "Ki Damah b'Sochah"?
Radak: Why was there all this punishment? Because its blood was inside. The blood spilled for no reason was exposed; it aroused the punishment.
What is "Tzechi'ach [Sela]"?
Rashi: It is smooth.
Radak: This is like Shen Sela (a peak), like "me'Acharei la'Chomah ba'Tzechichim" (Nechemyah 4:7).
What is the significance of putting the blood on a rock?
Radak: It is as if you put the blood on a high place, so it cannot be covered. The Mashal is, they spilled so much innocent blood that it is impossible to pardon it, as if the sin was hidden from Hashem. Rather, it is exposed in front of Him to avenge it.
Malbim: They caused it to be in front of their eyes constantly. I.e. they did not repent from their deeds, to cover their sins.
What is the meaning of "Lo Shefachas'hu Al ha'Aretz..."?
Rashi citing Targum Yonasan: You spilled [the blood] b'Mezid with a high hand - it was not Shogeg, to repent over it.
Rashi citing Yerushalmi Ta'anis 4:5: This refers to the blood of Zecharyah ben Yehoyada (they killed him in the Mikdash, and his blood boiled until the Churban about 250 years later). You did not conduct with (cover) it like blood of a deer or wild goat. Radak - the head killer [of Melech Bavel] saw it boiling, and killed myriads [to try to appease it] - "Leha'alos Chemah Linkom Nakam" (8).
