I was discussing with a friend about how could Saitama be killed. It's true he's basically a god in his universe, but that's more because of his incomparable abilities and power. But the fact of the matter is that the man is still a human. I'm not saying he can die from bullets or his skull breaking open, but others things that aren't as physical as we expect. I'm thinking the man can die if he's submit to constant radiation, giving cancer or leukemia. If there's ever a way he get cuts, he could suffer from an infection. Microscopic things he has no control of or definite ways to avoid. He can possibly be subject to psychological torture from a telepath or watching Boku no Pico for eternity. Shit, maybe poison soaked through his skin. It's speculation, but none of the monster we've seen so far have had poison or telepathy, and I feel like One is glossing over them because he knows those things can kill him.