Ellul chooses to say violence is a singular thing and leave no room for Christian participation in it. Yet he does contrast worldly violence against flesh and blood to "the violence of love" aimed at "the powers." He writes of this, "I remain convinced by Barth's and Cullmann's exegeses of the powers…" Without being familiar with Barth's or Cullmann's exegesis, I imagine he's identifying "the powers" as something along the lines of those in Ephesians 6, "the rulers, against the authorities, against the cosmic powers over this present darkness, against the spiritual forces of evil in the heavenly places."
So violence against "flesh and blood" is condemned, but as he writes, "to say that Christianity forbids all violence is not entirely correct. The Old Testament tells of a great many violences, though the greatest care must be used in interpreting these passages." (page 161) He briefly addresses three situations in the Old Testament in a footnote: Deuteronomy 2 and 7, Joshua 6, and I Samuel 15. It is worth reprinting the footnote in full:
"I give three brief examples. (1) The Herem [i.e., The Ban; see Deuteronomy 2 and 7, Joshua 6, I Samuel 15]. Obviously these passages are not to be taken as mandatory laws but as descriptions of an institution connected with a certain culture, whose significance alone concerns us. This is intended to mark the strict separation between the chosen people of God, who is holy, and the people who worship false gods. The atrocious Herem is intended to keep the people from idolatry.
(2) The prophets speak against the rich, but they never incite the poor to take justice into their own hands, to use violence. The prophets always pronounce God's judgment on the rich, they speak the word against the rich, but at the same time they declare that justice is the Lord's and that trust must be placed in him.
(3) Elijah slaughters the prophets of Baal. Three points must be emphasized here. It was not as a political or military leader, but as a prophet, and only after a miracle had been performed (thus in limited situation), that Elijah did his violent deed. He was struggling against the powers, the idols, the false gods, not contending for political justice or some human good. He stood alone, not only against the state but against the people also. He was working against the current. Only later did God reveal to him that some of Israel's people had remained faithful." (page 161)
Ellul views "the material struggle" as the unending cycle of violence—recall that he argues "violence begets violence." It is the spiritual struggle that brings real change. He writes, "People generally join the material struggle out of their own volition, spontaneously. They are able to conduct these political or economic wars, and if need be they will do it by violent means. But the other war can be waged only by Christians, for they have received the revelation not only of God's love but also of the creation's profound reality. Only Christians can contend against the powers that are at the root of the problem. The state would be powerless and unimportant were it not for the something-more-than-itself that resides within it. And to contend against institutions or against the men who serve the institutions (the police, for instance) is useless. It is the heart of the problem that must be attacked. And Christians alone can do that—because the others know nothing about all this, and because only the Christians receive the power of the Holy Spirit and are required by God to do these things." (page 164)
While materialists deal in violence, trusting it to deliver justice, it is only through Christians battling against the powers that will deliver real justice and end the self-perpetual cycle of violence.