As the cases of Digby and Smectymnuus illustrate, the Israel/Edom metaphor does not readily admit of middle ground. Indeed, in a famous sermon given on the occasion of a Parliamentary fast day on 23 February 1642, Stephen Marshall (the “SM” in Smectymnuus) argued “that all men are blessed or cursed according as they help or help not the church of God”—an idea leading him to conclude that “the Lord acknowledges no Neuters.” 
Accordingly, Marshall sets out in his sermon to stir up zeal among the problematically lukewarm. In service of this goal, he embraces the violent ending of Psalm 137 with gusto:
Now saith the Spirit of God, Blessed is the man, that thus rewards Babylon, yea, blessed is the man that takes their little ones and dashes them against the stones. What Souldiers heart would not start at this, not only when he is in hot bloud to cut down armed enemies in the field, but afterward deliberately to come into a subdued City, and take the little ones upon the speares point, to take them by the heeles and beat out their braines against the walles, what inhumanity and barbarousness would this be thought? Yet if this worke be to revenge Gods Church against Babylon, he is a blessed man that takes and dashes the little ones against the stones. 
Marshall seems to be saying that people unwilling to beat out the brains of “Edomite” children might not, in fact, be truly saved members of God’s Church. What’s more, his sermon seems designed to stir its hearers up to a state of zeal sufficient to commit such an act.
What’s interesting is that Marshall admits that this act looks like “inhumanity and barbarousness.” He assumes, that is to say, that his hearers are naturally adverse to such activity.
This natural aversion is where Marshall’s particular brand of Reformed theology comes into play: he believes that the Fall has left human nature so thoroughly corrupted as to make people in their natural state incapable of willing anything good. Thus, he treats reluctance to smash out children’s brains as, in this instance, a consequence of corrupt human nature that needs to be overcome by grace and replaced with godly zeal.
Marshall’s horrifying sermon becomes more disturbing still given the recollection that both Bishop Hall and Smectymnuus thought of themselves as Israel and the other as Edom. In the terms of this stark dichotomy, Psalm 137 becomes a recipe for unbridled, reciprocal violence.
And yet it’s also worth recalling that Hall, unlike Marshall, stops short of advocating actual violence. Even though he writes that his Edomitish opponents are “fit for darke lodgings, and Ellebore,” Hall does not create a scene of Grand Guignol out of the psalm’s final verses. In keeping with the same theology that motivated Marshall—the idea that God’s grace alone can overcome natural human depravity—Hall leaves dealing with the Edomites to God. Ultimately, he uses Psalm 137 only to suggest that his more intractable opponents ought to have no place in the debates about church government. He thinks that they should be ignored, not killed (even if his continued engagement with their tracts suggests that he has a hard time taking his own advice).
The most dangerous thing about Marshall’s sermon is that he speaks as though he were unaware that the bishops could use the same verses and the same rationale against him and his fellow partisans, when the logic of the psalm favors the bishops, and, moreover, when the Smectymnuan tracts published the previous year show that Marshall has read Bishop Hall using the psalm precisely as might be expected.
Is such self-satisfied literalism as Marshall’s the necessary link between violent words on a page and violent actions in the world? By giving this sermon at the time he did, Marshall was playing with matches on a powder-keg. Tensions between king and Parliament were already high following Charles’ attempted arrest of the Five Members the previous month, and Charles would in fact flee London out of fear for his safety a mere week and a half after Marshall’s sermon. After a spring and summer in which both sides prepared for war, the first pitched battle of the Civil Wars was fought at Edgehill in October. Tellingly, Marshall was present at the battle as the chaplain to the Earl of Essex, the Parliamentary commander.
Marshall could not have known what the future would bring, but he had to have suspected that violence was in the offing. In any event his sermon shows that he hoped the time for violence was near. Perhaps the Parliamentary setting of his sermon helped to persuade the king that he was no longer safe in London, and that no peaceful negotiation of his differences with Parliament was possible. Parliament ordered the immediate publication of Marshall’s sermon, which he apparently preached over sixty times. 
Marshall’s literalist invocation of the violence in Psalm 137 thus contributed a hefty dose of hellfire to an already heated situation. While war may have been all but inevitable, given the intractability of the problems and the tactical missteps on both sides, Marshall used scripture to hasten rather than to try and slow the onset of armed conflict.
Oxford Dictionary of National Biography, s.v. “Marshall, Stephen.” Cf. Paul Stevens, “Intolerance and the Virtues of Sacred Vehemence,” in Milton and Toleration, ed. Elizabeth Sauer and Sharon Achinstein (Oxford, 2007), 258.