[@orangebox]

I understand what you're saying--but note that Sylvia was not the one who directly killed them. She only witnessed someone die, and probably associated the infected man and John Marcel as people who weren't exactly 'human,' but as threats to her and her friend's existence. And Sylvia was in her cold, logical mood when those things happened--which is actually devoid of empathy and emotions. I guess society could consider her psychotic when she's in that state, but ironically, she is perfectly logical then. 

Edit: Yeah, they cuddled. Sylvia is like that. Her personality is dissociated--think of it as a midway between the average person and a person with Disassociative Identity Disorder (but not nearly as extreme). It's very common in society for people to have different personalities when dealing with different people or circumstances after all. She just takes that up a notch.