A.T. Robertson Commentary Revelation 16:6

"for they poured out the blood of the saints and the prophets, and blood hast thou given them to drink: they are worthy." — Revelation 16:6 (ASV)

For (οτ). Second causal conjunction (οτ) explanatory of the first οτ, like the two cases of οτ in 15:4.

They poured out (εξεχεαν). Second aorist active indicative of εκχεω with -αν instead of -ον.

Blood hast thou given them to drink (αιμα αυτοις δεδωκας πειν). Hαιμα (blood) is the emphatic word, measure for measure for shedding the blood of saints and prophets (11:18; 18:24). Perfect active indicative of διδωμ, and so a permanent and just punishment. Πειν is the abbreviated second aorist active infinitive of πινω for πιειν (επιον). It is the epexegetical infinitive after δεδωκας. There was no more drinking-water, but only this coagulated blood.

They are worthy (αξιο εισιν). "Terrible antithesis" (Swete) to 3:4. The asyndeton adds to it (Alford).