We went through this with Sis. I can remember her screaming at 5 or so that she was going to kill us all, and go live with friends/in the woods with the animals/in another country. It was a frequent comment.......but even after we'd get her calmed down---I was sitting with her on the top of our stairs once, and she looked at me and said, "But eeky, sometimes I really do want to kill people.". It's a hard thing to hear---and how could you punish that?
We treated it like any other "blow-up" and gave her the same consequences for misbehavior. Although it was ultra-shocking, those thoughts weren't her fault any more than a normal rage situation, was our take on it. Just talked to her a LOT about what EXACTLY would happen if we all died, and how it makes us feel, and why saying things like that made her look bad (she's always been really sensitive to appearing "crazy", and once she STARTS to calm down, saying that'll get her the rest of the way down).
We don't have any weaponry or, really, even sharp knives in the house. My biggest worry was that she would come up behind Mom or stepdad and push them down the stairs. Mom put her on Paxil a few years later, when it had gotten really bad, and we never heard about "killing everyone" again. Even once she weaned off, those thoughts never reappeared. Thank goodness.