The problem with that line of thinking is that you're really arguing that an entire technology is 'uninvented' because it could be used for evil. Suggesting that we ought to give up doppler radar entirely, with all its benefits like detecting victims in natural disasters, finding people trapped in fires, detecting criminal suspects, etc because it might be used by the next evil dictator is ridiculous. It's a completely unmeasured response. Besides being impossible to uninvent things, it just means we lose out on potentially useful technology now for an intangible benefit that might never come to pass.
The fact that the next evil dictator could have access to tools like this is reason to be vigilant not to let an evil dictator come to power and to be sensible about how we use these tools today to stop them being used in ways we think are unreasonable. It is not a reason to suggest we shouldn't have the tools.
That's absolutely not what parent was arguing, though. Also, that "line of thinking" not necessarily leads there. Accepting the technology is but one step. But we still need to raise moral and ethic questions about it. All parent was really saying is, that the nazis wouldn't have thought twice about using said technology to kill even more people.
It absolutely is what the parent is arguing. It's impossible to say "Evil people would use this technology for evil!" without implying something should be done to stop them, otherwise you're tacitly condoning evil. The thing that should be done is stopping the evil people from doing things, regardless of what they use, rather than questioning morally ambivalent technology just in case evil people use it.
Once a technology has been invented, uninventing it because it might be used to harm people doesn't work. Especially if the people wanting to do the harm are in a position of power (like the nazis were).
Wondering about the countless wonderful or horrific uses new technologies might have is simply part of critical thinking.
I don't think anyone did that calculation but if we were to add up all the good technology brings us with all the bad it brings us, I'm not sure we'd end up in the positive.
So I think it's pretty natural when you hear about some new technology to ask yourself "great, how's that thing going to be used against us?"
Asking yourself what you could do to stop it is a completely different matter. And we all know here that security through obscurity is a pretty poor defense.
The fact that the next evil dictator could have access to tools like this is reason to be vigilant not to let an evil dictator come to power and to be sensible about how we use these tools today to stop them being used in ways we think are unreasonable. It is not a reason to suggest we shouldn't have the tools.