I have no problem with saying it. While any unlawful death is sad and unfortunate, no single death is worth more from a societal stand points than the rights of the citizenry and the legitimacy of the institution of the Constitution. As such, while unfortunate, things like deaths due to someone misusing a firearm, criminals getting away with murder because of the 4th and 5th amendment, or terrorists propaganda being able to be spread because of the 1st are the unfortunate costs of the freedoms we enjoy in this country.
I absolutely view mass shootings a potentially valid reason for a form of gun control, within the context of the incident in question and the form of gun control being called for. You seem to be under this assumption that if one does not agree with all forms of gun laws, or does not agree with the specific types being offered, in the wake of a SPECIFIC one of these events then it somehow means they are against ANY form in response to ANY such events.
No, most pro-gun advocates are not in favor of utilizing a mass shooting to pass a gun control regulation that would not have had any direct impact on the particular mass shooting in question, because in such an instance it's clear that the mass shooting is just being used as a cudgel to beat an already present political point. And no, most pro-gun advocates won't be in favor of certain wide ranging and substantial forms of gun regulation that they believe violate the constitution in response to mass shootings. But that's not the same as the broad claim you just made.