i feel like the niners/cards rivalry was much more heated when the cards were good and we were bad because at that time our team was feisty and would beat the cards most of the time, but never manage to win the division. that is what can cause hatred. but when one team is winning the head to head AND winning the division then it makes it difficult to have any type of hatred.
i actually don't hate the seahawks, we've beaten them the majority of the time since cheatin' pete has come around (even with dingleberry!) but i don't like to see cheating teams in my division so that is where the majority of the animosity comes from.