DAVID: I used protomatter in the Genesis matrix.
SAAVIK: Protomatter. An unstable substance which every ethical scientist in the galaxy has denounced as dangerously unpredictable.
DAVID: But it was the only way to solve certain problems.
Yes, ST III makes David take the blame and responsibility. Still, according to ST II, his mother was in charge of the Genesis Project, and I have a hard time imagining her not to know
what materials had come into use for the Genesis Matrix.
That was her responsibility and ST III made her look incompetent in a debatable retcon manner.
I sympathize with the efforts for rationalization of this mess, but this was really bad script writing, apparently as an excuse to have Spock age during the movie's running time and to have protagonist and antagonist fight it out on a planet that's falling apart.