Joel 3:21

And I will hold as innocent (R.V. marg.) their blood which I have not held as innocent] By the desolation of Egypt and Edom, Jehovah will shew openly that the murdered Judahites (Joel 3:19 b) had suffered innocently. So long, namely, as He permitted their blood to remain unavenged, it might be supposed that they had not been slain unjustly: but by the punishment of the murderers (i.e. here, by the desolation of their country) Jehovah declares (implicitly), what He had not declared before, that their blood was innocent (Joel 3:19 b), and had been unjustly shed. Niḳḳâh, to holdor declare innocent(Exodus 20:7; Job 9:28 al.), is chosen on account of nâḳî", innocent, in Joel 3:19.

and Jehovah dwelleth in Zion andis here almost equivalent to as truly as:in corroboration of the promise made in the preceding clause, the prophet appeals to the indisputable truth that Jehovah's dwelling-place is in Zion. So Hosea 12:5 "and(as truly as) Jehovah is the God of Hosts, Jehovah is his memorial (name)."