Those Slain by YHWH Shall Be Many
Isaiah 66:14, " fire YHWH will enter into judgment, and by His sword, with all flesh; and those slain by YHWH shall be many."

I tend to emphasize the revelation of God's identity at the cross--as I think the authors of the New Testament do as well. However, the Name of God revealed in the crucified Christ must be a lens through which to understand all of revelation...not a filter by which certain aspects of revelation are dismissed. YHWH is the same, yesterday, today, and forever, and these texts (see also Jer.25:33, Rev.6:16, 19:21, etc) about the coming mass execution of many cannot be dismissed. Jesus Christ will slay many upon His return, and those pierced by the One who was Himself pierced will cover the planet. This is true.

So--how are we to understand the slaying of enemies at the hands the God whose glory is definitively revealed in a substitutionary death for His enemies? I have two answers, one reasoned out based on scripture, and one a refuge of faith and admission of mystery.

First, If we have accepted the crucified Christ as our Lord and God, then the wounds of His death are our security and heralds of His love, they bear witness to redemption before the throne and--upon seeing them--we will, as it were, run to and hide within them until the storm of wrath has passed. 

However,  for those who have rejected and hated and staunchly opposed the crucified Christ, then the wounds--wounds in God's own body--declare the magnitude of their sin and the infinite degree of their rebellion. They are eternal witnesses to the deicidal passion of the human heart and are thus witnesses against them in the courts of heaven.

The return of the King will be glory and splendor for His people and fire and judgment for His foes (2 Thess.1:6-10). Christ's wrath absorption on the cross becomes the basis for His Bride's salvation and the basis for His own outpouring of that same wrath upon those who cling to the bestial gods of the world and refuse His mercy (Rev.6:16). 

However, even with the above explanation, someone might still say, "but if God's glory is seen in His absorption of wrath and giving of Himself to make His enemies into His children, and if all of reality exists to communicate His glory, would He not receive more glory if He absorbed all the wrath and turned all the enemies into children?" If we were going merely by human reasoning, that would seem to make sense....but we our reason must always plow under the yoke of faith in God's revealed word, and the simple fact is that there is a final judgment and we are told many will be condemned. do we answer this interlocutor? My bedrock response is simply to bow the knee to divine revelation and trust the Name of our look at the fires of judgment and the cross of the Son and put my hand over my mouth and confess, "My Lord and my God, I trust you to do what is best, I trust the Name you've declared at the cross, and I know that Calvary's heart will wield Armageddon's sword with infinite justice, wisdom, love, and beauty." 

...His work is perfect, for all His ways are justice. A God of faithfulness and without iniquity, just and upright is He.