Not from salt but you could make it from NaOH and magnesium there are many tutorials.
As for the most toxic thing you can make at home, I'll go by LD50 and not necessarily by the usefulness as a weapon or something. The protein in rosary peas or castor beans (ricin) is incredibly toxic. But it's not very interesting as a chemical and you won't extract a practical quantity (which is why I'm sharing this) if you want to actually synthesize it id just go through the LD50 lists and pick one. It could make for fun organic chemistry actually and is a really safe exercise if you pick something non-volatile and only make a tiny quantity.
