What will arouse anger?
Rashi from Yerushalmi Ta'anis 4:5: I commanded that the blood will not be absorbed in the ground, in order that [I] will remember to avenge it.
Radak: They greatly murdered openly, in order that His anger will be against them to take vengeance.
The previous verse said that they put it on the rock. Here it says that Hashem did so!
Radak: [Yerushalayim] increased the blood in order that it will be exposed, as if it is on a smooth rock. Also I made it exposed, lest it be covered; I did not want to pardon, for the sin was too great to bear. So it says "va'Ymalei Es Yerushalayim Dam Naki v'Lo Avah Hashem Lislo'ach" (Melachim II, 24:4).
Malbim: This was putting the bones under the pit to increase the fire.
