Having said that, if you happen to have purified ricin, like our heroes do after Walt prepares it, then a dose about the size of a grain of salt could be enough to kill someone. Walt can either cause his victim to breathe in the dust or eat/drink it or inject it somehow. You don't immediately keel over dead from ricin poisoning. A few hours after exposure, you would start to feel very sick. Your symptoms would depend on how you were poisoned. If you breathed ricin, you would start to cough, feel nauseous, and find yourself short of breath. Your lungs would fill up with fluid. Low blood pressure and respiratory failure could lead to death. If you ate or drank the ricin you would suffer cramping, vomiting, and bloody diarrhea. You would become extremely dehydrated. Death would result from liver and kidney failure. Injected ricin would cause swelling and pain in the muscles and lymph nodes near the site of the injection. As the poison worked its way outward, internal bleeding would occur and death would result from multiple organ failure. Ricin poisoning is not easy to detect, but it is not necessarily fatal, even though it is unlikely medical staff would identify the underlying cause. Death usually occurs 36-48 hours after exposure, but if a victim survives a few days, he has a good chance of recovering (though he will almost certainly have permanent organ damage).
So, those are Walt's options for his ricin. If he uses the poison, it isn't likely he would get caught. Ricin poisoning isn't contagious, so he probably won't harm anyone except his victim, though carrying around a potent toxin is a bit risky when you are dealing with druggies who sniff everything that comes in a small bag. It will be interesting to see what happens.
Breaking Bad Chemistry | Ricin Fact Sheet