"Our hands have not spilled": Now would it enter our minds that the elders of beth-din are spillers of blood! (The intent is:) He (the slain one) was not "in our hands" and dismissed without a meal; and we did not see him, (corresponding to "our eyes have not seen") and let him go without an escort.
And the Cohanim say (Ibid. 8) "Forgive Your people, Israel, which You have redeemed, O L-rd."
"which You have redeemed ": This teaches that this atonement atones for those that left Egypt.
These are the dead, whereby we are taught that the dead, too, require atonement, and that this atonement atones until the exodus from Egypt, and that the spiller of blood sins (retroactively) until the exodus from Egypt.