Look, it comes down to this:

Is there something innate about our intelligence that makes it impossible to match except through human brains? That strikes me as something akin to spiritualism. The answer is almost certainly "no." We're not talking about warping space through some hypothetical state of matter with laws of physics different from our own. We're talking about at least human-level intelligence, which is something we already have an existence proof of (us).

And humans are the most dangerous force on the planet. No animal (beside maybe microscopic organisms) stand a chance against a group determined humans. We're nearly unstoppable due to our intelligence. Our we so unique? Is it so impossible that machines could some day (perhaps in our lifetimes) be built which achieve human-level intelligence? Considering the computer advances that have already been achieved, it is clearly a real possibility if not a certainty.

Human-level intelligence is perhaps the most powerful (thus dangerous) thing on this planet. Making a machine that is at least as intelligent (and perhaps much more so!) is clearly something that could be incredibly dangerous, and we know such a thing is physically possible, unlike many things in science fiction.