Ps. 89.43 (David into Saul)

You have even turned back on himself / the edge of his sword
and failed to support him in battle. 

The previous verse contemplated the ‘external’ act of Yhwh in relation to the king’s enemies—he ‘strengthened’ their hands. Here, that aggression in turned ‘inward’ on the kings themselves. They are made to fail ‘in themselves’—their own swords are turned back upon themselves. Again what we are finding is the utter vulnerability—the absolute vulnerability—of the kings as they now begin to implode. They are not merely ‘surrounded’ by Yhwh’s aggression; they are consumed by it. There is nowhere to turn as every aspect of their authority and power is now being turned against them. This is not just a vision of the-king-without-Yhwh, but the king who is being assaulted by Yhwh, as he extends his sovereign control over the king into the king’s own inner ability to fight his enemies. 

It is as if what we have here is Yhwh’s treating of David as he treated Saul. One wonders, in fact, whether this ‘turning of the sword’ could be a reference to Saul’s falling on (or being driven through with) his ‘own sword’ as he stood on the failed battlefield, in the presence of the aggressively absent Yhwh.