Don't they already do that?
Every time something happens MMM and Sarah Brady trot out their litany against guns, but they try to disarm everyone.
How many times do we hear about the loonies who go postal and afterwards their acquaintances all remark about how they knew it was going to happen and yet they did nothing when they could have intervened.
We sure don't seem to have a problem intervening to protect property by killing an intruder on our property do we?
They try. Codifying a government's duty to disarm potentially-dangerous people would cause them to do it a lot more.
Every time something happens MMM and Sarah Brady trot out their litany against guns, but they try to disarm everyone.
Yeah, Brady et al. want to disarm everyone, but do you think they would have any objection to disarming the populace piecemeal?
How many times do we hear about the loonies who go postal and afterwards their acquaintances all remark about how they knew it was going to happen and yet they did nothing when they could have intervened.
And how many more times are there that people have such suspicions but nothing ever becomes of it? Of course, you never hear about those times, because nothing happens worthy of reporting (or even, in many cases, particularly remembering).
BTW, another point to consider: if people can only be deprived of liberty as a consequence of prohibited overt actions, then someone who thinks he might have some mental problems but wishes to retain his liberty would further his interests by seeking help, so as to avoid performing such actions that would cost him his liberty. If, however, people may be deprived of liberty as a result of perceived risk, then those who value their liberty would further their interests by avoiding any actions that might increase the perception of risks, even when such actions would reduce the actual risk.
People occasionally go off the deep end and do very bad things. That is unfortunate, but inevitable. Policies to act preemptively against such people before they commit any prohibited overt act will almost invariably (if not inevitably) have side-effects that are worse than the problem they are supposed to solve.