Archie, though, had nothing to do with the cause of death (or, if Charlie ended up drinking at least some of the poison, that needs to be spelled out more plainly). If you lay out a chain of events leading to Charlie’s death, and remove Archie from that chain, absolutely nothing changes.
As another thought exercise: Bob shoots Charlie several times, and he’s dying, and then Archie snatches the gun from Bob’s hands, holds the gun to Charlie’s head, and pulls the trigger, but the gun is empty. Charlie dies from the gunshot wounds. Bob is guilty of murder. Archie is guilty of attempted murder.
What Bob did killed Charlie. What Archie did would have killed Charlie, if not for Bob’s actions.
Red herring. We’re all going to die eventually. Intentionally causing someone to die sooner than they would have otherwise is murder (excepting certain, very limited, circumstances). If I shoot someone on Death Row heading for a firing squad, that doesn’t absolve me of the murder.
That’s felony murder, which is a whole different animal. That is, if you commit a “dangerous” felony (like robbery), and someone dies as a result of that felony, you are guilty of felony murder. No one died as a result of Archie poisoning the flask, so he’s just guilty of the attempt.
The only crimes being committed are the attempts at murder. Archie’s failed. Bob’s succeeded.
See my earlier analogy of throwing someone out of an airplane that’s doomed to crash. You’re trying to kill them through dehydration. They died of dehydration due to your actions (even if they would, otherwise, have died of poisoning). How is that not murder?
ETA:
Actually, my first reaction was “Charlie killed his own damn self when he didn’t check his canteen for a leak, patch the leak, and refill it before splitting up. You’re responsible for your own gear.”
