Two different offenses
Poisoning may seem like murder since the expected result is the same: death. It is the result of a historical fear of regents. All regents under guard could not ignore the risk of poisoning. This is why it is incriminated independently in the penal code.
However, the autonomy of this offense has made it special by dint of interpretation to differentiate it from assassination, which is premeditated murder.
The intention to kill
Murder and poisoning share the same mental element, or intent of the offender. Both the murderer and the poisoner must have intended to kill. Note that the error about the victim is irrelevant. Concretely, if the poison or a shot kills a person other than the one targeted, the judge will still retain the intention to kill.
The main difference: the modus operandi
While the failed murder will not be considered as such, but will be called an attempt by the judge, the same is not true for the poisoning. The crime of poisoning is not consummated, that is to say actually committed, by the death of the victim. Poisoning is committed as soon as the perpetrator has given the victim the substance capable of causing death. Concretely, in the event of a poisoned dish, the author will be guilty of poisoning as soon as the dish has been served to the victim, even if the victim has not ingested anything.