The human body contains dangerous chemicals and other hazardous stuff. Stomach acid is very corrosive. Immune cells are adept at destroying, and can analyze their targets to find the best approach. The brain on its own is a very harmless organ, but give it something to control and it can do more than simply cause harm to anything around it, with the only thing keeping it from destroying worlds being morals, which are absent from it when its exposed to any of various neurotoxic chemicals. We should worry less about machine takeovers and worry more about literally anyone or anything that ends up going rogue, before our entire God's-Gift green planet of Earth burns in manmade Hades.