I mean, how the heck was David supposed to know that if he had used Protomatter to get Genesis working that it would cause the Reliant to not notice that Ceti-Alpha Six exploded and that Ceti-Alpha Five was now a barren waste land? There is no way he could have known, so why pass judgement on him?
He wasn't, but that's not the point. The crux of Saavik's argument is that David acted rashly by incorporating the proto-matter into the Genesis matrix. A more responsible scientist might have waited until a safer option were available before proceeding with the Genesis experiments, even if meant putting the project on hold for months, years, or longer if that's what was necessary to find a better solution.
Instead of waiting, David decided to use the proto-matter. In doing so, he set in motion a chain of events that led to Khan's escape for Ceti Alpha V, the theft of the Reliant
, the death of Spock, the destruction of USS Grissom
, the theft and destruction of Enterprise
, David's own death, and the death of the BOP crew. Did he mean for each of these things to happen? Of course not. But his behavior paved the way for the events that encompassed the creation and demise of the Genesis planet.