Despite some very bad dialogue...
Saavik: How many have paid the price for your [impatience]? How many have died? (Uh, none.)
None? If David had stuck to using approved means, the project would never have gotten as far as it did, Khan wouldn't have been "rescued" and none of the scientists on Regula 1 would have been tortured and murdered. Khan wouldn't have attacked the Enterprise and killed Peter Preston (I assume other cadets and crew were lost in the attack).
While Khan and his people dug their own graves, they would have remained alive on Ceti Alpha 5 had David not used protomatter.
Without Genesis getting as far as it did, there would be no new planet for the USS Grissom
to monitor. The Klingons wouldn't have had a reason to meet and destroy the science vessel, killing everyone aboard.
David has the blood of well over a hundred people on his hands. Sure, Spock came back, but are the needs of the one really more important than all those dead people?