This is just speculation on my part, of course, but I think if they'd gone through with the original plans to have Saavik be the traitor, David Marcus' death wouldn't have had much to do with her motivations. Any Saavik/David flirtation was pretty thoroughly edited out of TWOK, and despite what the novelization depicts, Saavik and David don't have any romance or special relationship going on in TSFS either.Well you don't like/trust them either and They killed your son!, and that also would have felt too small universe that both reluctant-but-reforming Kirk and the actual murderous traitor's main motivations directly stemmed from the exact same incident (that we saw, had been big part a few films ago).
Valeris' "They killed your son" line is about her trying to twist the knife to Kirk, not about her own motivations, and I feel like it would've been played the same if it had been Kirstie Alley's Saavik delivering those lines.