Bandit LOAF
Long Live the Confederation!
You're remembering correctly, but I'm fairly sure that idea originated in the novel rather than the script.
Probably all of them. If Genesis weren't something that could double as a weapon then neither Khan nor the Klingons would have been especially interested in it. He made the decision that ultimately lead to others killing the Regula One and the Grissom crews.
Sorry to go back on this one, but rereading it... it still seems inappropriate. I mean think about it, most of the greatest inventions to date have either stemmed from, or become weapons (Nuclear power, horseless carriage, Radar, sonar, rocket tech, space satellite, etc.).
Still to place blame on him for the deaths of the people on Regula, the cadets who died during the fight against Khan, and the crew of the Grissom, is pretty extreme. David wasn't the head of this project, his mother was for one. For two this was never intended to be a weapon, in fact it seems that David wanted Starfleet to have nothing to do with this project and went to great lengths to make that clear.
(Also, it's worth pointing out that while we know Carol Marcus exists, Star Trek III as a stand-alone story doesn't. The movie removes any references to her and treats David as the creator of Genesis. It's no coincidence that they refilmed the 'explaining Genesis' tape with Kirk instead of Carol...)
Yeah I always wondered why... In WOK it seemed like she was all important while David was a mere assistant, who was resentful of Star Fleet involvement.
In the end, "you Klingon bastard, you've killed my son" sounds a heck of a lot more dramatic than "you Klingon bastard, you've killed the mother of my son whom I am not otherwise involved at the moment."