For reference, the Amanita genus of mushrooms is responsible for the majority of mushroom poisoning deaths, and the prime reason why people tell you not to eat wild mushrooms. (Several species look just like the classic "button" mushrooms you can buy in a grocery store.)

It's also not an immediate agony from what I've read, which is exactly why it's so deadly. It's possible to save someone who gets prompt treatment, but most people have no idea they ate anything poisonous until it's too late. What the poison does is it actually attacks your RNA, making it so that your cells cannot properly divide and repair the body's wear and tear. Symptoms don't show up for half a day or so, and before that, there's no warning signs. By the time you notice anything is wrong at all, the poison has already destroyed your liver and kidneys and is well on its way to causing systemic organ failure, and you're a dead man walking. (Well, or dead man lying down and writhing in agony.)