Last time we dealt with David accepting responsibility for the massacre at Nob and ה׳ making him decide his own punishment, and how he ended up almost losing his descendents:
We have a פרק תהילים where David prays for them:
שמונה מי יודע
Rashi above connects the שמינית with the eight generations until the atrocity of עתליה. But I think the idea of a שמינית goes deeper than that. There are two תהילים that are על השמינית: our פרק, and פרק ו, ה׳ אל באפך תוכיחני; ואל בחמתך תיסרני. Note that we associate the latter with David’s statement to Gad that we quoted before (שמואל ב כד:יד) ויאמר דוד אל גד צר לי מאד; נפלה נא ביד ה׳ כי רבים רחמו. Both of these associations are with the incidents in which (either explicitly or in the eyes of חז״ל) David chooses the consequences of his actions. The number seven in תנ״ך symbolizes “everything in the world”, as in the seven days of creation. Eight symbolizes the step beyond that—the active involvement of the human being in the world. That is one explanation of the ברית מילה on the eighth day. A psalm על השמינית is one that reflects man acting as the agent of הקב״ה, even to the point of deciding his own punishment.
Here David asks הושיעה ה׳, not הושיעני ה׳ the way we would expect from the rest of תהילים. The entire פרק is impersonal. David is not praying for himself but for others, since he includes himself in the פסו אמונים מבני אדם.
שפת חלקות
Reading this פרק in the light of Rashi’s interpretation makes it more wrenching; David is describing himself as the לשון מדברת גדלות, the smooth-talking boastful one. His lies are what doomed אחטוב, his family and all of Nob, and it is his descendants that will have to endure the punishment of כסף צרוף בעליל לארץ. And it ends on a kind of down note; the world is still כרם זלות—the base (זֻל) is considered like the exalted (רֻם). All that David can hope for is that ה׳ will guard the innocent.