I realize this is a comparison for clarification, but isn't it still just 'categorically demonizing innocent people'?

You picked a more controversial topic where more people are likely to agree with the demonization I suppose, but your assertion that the power to kill "may be considered unjust by principle" is not well supported by vague assertions that "some think" guns "may be seen as" unjust instruments of violence. I fully support everyone's right to object to something they see as dangerous; opinion does not constitute principle, however.

To me, something is just or unjust based on whether or not it aligns with or infringes anyone's rights. So, I suppose I might actually agree that a power could be "unjust in principle" if it could be shown that the power could not be used justly - that is, without infringing on anyone else's rights. For some powers, mostly political ones, this is the case. In this case I think guns may be a poor comparison, because they actually can be used in ways which are just (defense, etc.), even if you believe that those cases are unlikely and so desire strict gun control, etc.

In contrast, it may be that producing nonfree software is always 'wrong' (in that it infringes on the supposed rights of the users to understand and modify the program they are running) and therefore having or providing the power to do so would be 'unjust in principle'. If the concern is merely that some may produce malware, and there are actually legitimate reasons for producing nonfree software, then it is not unjust in principle to do so, or to provide someone with said power.