I think what you are missing here as far as the argument goes is what is the process for putting people on the terror watch list? Its a secret and we all need to know the criteria. Otherwise the govt could put every combat vet and every tea partier who ever made a cryptic comment on social media on the terror watch list and voila’ you have millions denied their rights under the second amendment.
For me it doesn’t matter. Being placed on a list is not the same as a conviction of a crime and does not require the same burden of proof (proof beyond a reasonable doubt) nor due process. There is no fair trial, no presumption of innocence until proven guilty. It is simply a secret government list.
There are many, many well documented cases of people being put on the list wrongly and then having an exceedingly difficult time getting off of it. What’s more, it would not have solved a single thing in any of the previous acts of domestic terror. Not at all. It is simply a feel good measure designed by politicians to make it look like they are doing something.
I have several conservative friends who support this like Mr Trump, and it boggles my mind. It is like they don’t understand the concept of rights. When asked if they would support people being on the terror list losing their right to unreasonable searches or seizures, their right to speak freely, their right to vote, or the right to be protected from cruel and unusual punishment, they say “of course not.” Why is it different when it comes to this right?