We have a tendency to try to make soap characters into black & white heroes and villains when really it's not a black & white medium.
Sarah died. That was devastating. She freaked when she saw her messy mouthed brother after his first bite and caught a bug as a result. If Barnabas had been eating a sloppy joe and got the stuff all over his mouth, took a walk and Sarah saw him, I don't think she would have had the same reaction. The story was for her that Barnabas went away and that was that. A story invented because he was, in fact, dead. Why was he dead? Okay, why was he messy? I guess the first bite was a bit of a trauma for him, along with the whole blood-letting thing. He didn't clean up. The whole thing can be see as a tragic chain of events.