GP's point (and I agree) is that an evil actor could, in principle, do those things already (and a much more intelligent one, while we're at it).
And yet, it doesn't happen. Why? Because reality is not a "winner takes all" game. Actions carry consequences and other factors limit the extent of damage any actor could do.
It is way more threatening to human life that there exists, today, a huge stockpile of nuclear weapons all around the world.
Nah, you'll be surprised at how dumb things in the real world could be; like, the nuclear briefcase password is probably 0000-0000.
Punishment and retaliation is what deters most people from going rogue. Note that by retaliation I do not mean "fear of retaliation", I mean the actual consequences that come to you from FAFO in the real world.
Imagine an AI agent which manipulates exposed SCADA systems. That could be incredibly destructive!