Depending on how you look at him, Nestor-10 is either a practical joker or a serial killer in the making. And the fact that people can't quite decide on what he is leads to the conflict among the humans, including Kallner's threat to arrest Susan Calvin (224). Which just goes to show you: robots may be dangerous, but humans are the real problem for Susan Calvin.
Nestor-10 is one of the modified Nestors that help in the research on Hyper Base. That is, Nestor-10 has a modified First Law: he can't hurt people, but he can let people get hurt. (So, like, he couldn't trip a person, but he could let a person fall down.) But that's not all. As Calvin explains, the fact that some Nestors have modified First Laws means that their brains may be unstable (65). Which means that they might one day be able to break the part of the First Law that they are imprinted with. (So, they could start tripping people.)
Nestor-10 is the closest thing to a killer robot that Asimov gives us in these stories. But how close to a killer is he? Well, even at the end, when he's pushed to his limit, Nestor-10 can't attack Susan Calvin (322), so he doesn't look like a killer. But Calvin says he might eventually have overcome the First Law, and Calvin is never wrong. (She may not know the answer at first, but she always figures it out.) So, maybe it's a good thing that the humans destroy all the modified Nestors at the end of this story (325).
Then again, are the Nestors really the problem here? Think about it: Nestor-10 may be dangerous, but part of the problem is that Gerald Black told him to get lost. Or we could go even further than that: part of the problem is that the government and US Robots made the Nestors unstable, knowing that they were unstable.
So, let's be honest here: Nestor-10 may have become a killer, but if he had, it would've at least partially been the humans' fault.
What does this robot tell us about robots? Even a rebellious robot can't really rebel in a dangerous manner (at least, not at first). And even with a dangerous robot, part of the problem is the human element.