Normally, making a poison is not in and of itself a crime. If a third party took the poison from the person who manufactured it without their knowledge, the manufacturer would generally not have criminal liability, at least in the absence of "gross criminal negligence" such as leaving the poison manufacturing location totally unsecured and letting people know that there was poison there for the taking.
In a civil case, someone might sue the poison manufacturer for negligently securing their facility, but again, that would be a real stretch if even ordinary precautions (e.g. standard locks on doors and cabinets) were in place, or if it was an inside job theft.
In the same way, a gun store owner is not usually liable criminally or civilly if someone steals a gun from his store and shoots someone with it.
The police could certainly charge Person A with capital murder mistakenly believing him to have intended to kill and did kill someone with the poison, which would make the critical factual point establishing that Person B gave it to someone without Person A's knowledge.
Person A might still be guilty of attempted murder if he intended to kill someone (not necessarily the person who was killed) with the poison but had not fully carried out the plot when the poison was stolen.
Some places probably require a permit of some kind to make poisons, and if Person A didn't have a permit, he could probably also be charged with making poisons without a permit.