16:6for they poured out the blood of saints and prophets, and Thou hast given them blood to drink. They deserve it."
for they poured out the BLOOD of SAINTS and PROPHETS, and
It is interesting that PROPHETS are included here. Probably this refers specifically to the two witnesses who were recently slain (Revelation 11:10).
In a broader sense, this may be the "severer punishment" referred to in Hebrews 10:26-29-31.
THOU hast given them BLOOD to drink.
It is on purpose that those who took the mark of the beast are "forced" into drinking blood to keep alive. We, as believers, have celebrated Christ's BLOOD all these years in Holy Communion. Now this other kind of BLOOD must be drunk. And those with the mark of the beast must have drunk (see Revelation 17:6). This must have been especially difficult for practicing Jews who had the mark for they are not to drink BLOOD (Leviticus 3:12-17).
This is group vengeance. That is, individually a person may not have shed a SAINT's or PROPHET's BLOOD, but by virtue of the mark of the beast that they wear, they are forced to take God's vengeance. The rules of Deuteronomy 19:15-21 are in effect again.
Note that the request of the first half tribulation martyrs finally is fulfilled (Revelation 6:11). With the alter itself confirming that the wait is over (next verse).
THEY DESERVE IT.
Isn't it interesting that in the selfish society we live in today, people feel they DESERVE or have RIGHTS to such and such. This will be one time they wished they weren't so demanding......I can hear them now, "What did we do to DESERVE this?"
Well, you took the MARK OF THE BEAST, and the period of GRACE has ended. You now are experiencing the retribution and judgment created by your decision to take the MARK OF THE BEAST.