It is already very plausible (and has been since the 1950s) without the advent of LLMs. This is just another layer on top of the preexisting and very plausible existential threats we already face.
Detail it. Justify it.
Your comment about before LLMs is a non sequitur. Demonstrate that an LLM can kill everyone on the planet.
Detail it. Justify it.
Your comment about before LLMs is a non sequitur. Demonstrate that an LLM can kill everyone on the planet.