Oh I thought about your reasoning. It just need to keep the one that asked the question, then. Killing everyone else won't prevent it to tell the answer to the researcher.

Even with limited hardware, the AI still can have a non-trivial influence over the world, provided it can self-improve. And if you limit the hardware to present-day computers, and you ensure the format of the answer is highly constrained, you may be safe. The AI will be much more limited, though.