Seest the reins and the heart: the prophet had, Jeremiah 17:10, spoken unto God under this notion; here he appealeth to him as such, who therefore must needs see as well the prophet's sincerity as his enemies malice. Let me see thy vengeance on them: for his prayer against his enemies, it is of the same nature with what we met with Jeremiah 11:20, indeed this whole verse is the same with that. David, Psalms 58:10, prophesieth that the righteous should rejoice when they see the vengeance which God brings on his enemies. But yet the prophet seems in this petition to have showed himself a man (as the apostle speaks of Elijah) subject to like passions with other men; for although God sometimes by his providence causeth vengeance to come upon his and his people's enemies in their sight, yet whether they may absolutely pray for it is a question. See Poole on "Jeremiah 11:20".
Related Commentaries of Jeremiah 20:12
Jeremiah 20:12
12 But, O LORD of hosts, that triest the righteous, and seest the reins and the heart, let me see thy vengeance on them: for unto thee have I opened my cause.