Why Didn't Claire Use Her Blood to Save Nathan?

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Remember in season 2 how Claire and Adam's magical healing blood was all the rage? Well, what happened? Did the writers expect us to forget that ever happened? I'm really getting tired of all the plot constructs on this show that insult our intelligence. Claire could have easily saved Nathan with her blood after Sylar killed him (even though I wanted him to die anyways) and they could have just killed Sylar (even though I like Sylar alive). Why do the writers/producers continually assume we're idiots?