What, exactly, are rights? Someone's idea of entitlements... a contract with your government, that basically says, "this is what I am expecting you to do for me... this is why I allow you to collect taxes from me, this is why I engage in otherwise socially acceptable behavior."
Typically, if you live in a region under a government, the list of rights in said government's constitution is what you get: take it or leave it. Of course, in many places (certainly the US) there are all kinds of arguments about what our rights mean, whether the government is truly enforcing them and (if not) what to do about that.
Why ask? Well... turns out the Orlando shooter was a security guard. I don't know much about G4S, Mateen's most recent employer, but I do know there are plenty of "security firms" where people such as Mr Mateen are hired, trained to use firearms and placed to protect people and property. Presumably beyond "a well-regulated militia," G4S wouldn't have allowed Mateen to just walk out of a place of employment with a firearm, even if he had parted ways favorably with his employer. Which he did not.
But I am inclined to assert that, as a member of the security guard industry, Mateen was going to get a firearm. It's silly to debate firearm regulation with him as justification, because (unless you are arguing that US police and security personnel shouldn't be allowed to carry firearms) Mateen was going to get what he wanted.
Side observation: if Mateen was well trained (frankly, I have no idea whether he was or not), he would have been able to inflict the casualties he inflicted with a bolt action rifle. He didn't need a semi-automatic in that environment and certainly didn't need an automatic. I daresay Miyamoto Musashi would have easily been able to achieve the same (or more complete) result in less time with two samurai swords.
Then there is the question: did Mateen deserve to be separated from G4S?
Most employers in the US offer "at will employment." There are contracts out there (I'm always fascinated to see these...), but just about everyone works under an "at will agreement." You can leave, your employer can ask you to leave. It's a "no fault" situation: you can quit without cause and your employer can ask you to leave without cause. Hopefully, you or I can easily move to a new situation.
In high school, little girls are marketing themselves against rivals (I know this because I recently transported some HS girls to an amusement park and got to listen in). Personal brand rivalries exist everywhere: in schools, professions, even churches. Some employers do NOT want an employee to leave, because it "harms the brand." Yes, there are employers who will fire you if they think you are interviewing... Vicious marketing is happening between Donald Trump and Hillary Clinton, at the moment. Welcome to America (or perhaps I should say, welcome to the human race).
When I was in business school, some employers (all?) would "war game" who would accept an offer and engage in the "offer dance" because they didn't want any offer going unanswered.
Why is this important? Because in the "personal and professional brand marketing game" there are winners and losers. People get paid more if they are winning. Companies are more profitable if they are winning. And the truth of the matter: marketing is a spin game. Lies, or at least half truths, are being told, implied... in some cases viciously propagated.
If you lose that game, you lose friends, income... the list goes on and on. Somehow Mateen lost that game. He was religious, so his religion became his excuse for aligning himself to ISIS... but ISIS hadn't hired him (although after the fact, was willing to accept credits).
Someone hired the guy and trained him. And, perhaps if he was still working, that terrible tragedy may not have happened. There are malcontents and criminals: we need security guards to protect against those. Was Mateen a dysfunctional criminal? Should someone have picked up on that, and left him unemployed from the "git go?" He wouldn't have done the damage he did... but what a conclusion.
What is the key here? What is the due process, that if the FBI, or a psychologist, or someone's wife/husband deems you a threat, that you can be prevented from owning a firearm? These are details we need to sort out... we have substantial gun legislation in place that should have stopped many of the mass shootings that occurred, BUT... if a person violates a fundamental prerequisite for his or her right to bear arms, an observer needs evidence/justification for taking action and a robust process for moving forward (read: placing someone on a "don't buy list").
So, of course we have to ask: the FBI interviewed the man and concluded he wasn't a threat. Why is that? Apparently Mateen was a devout Muslim and even traveled to the Mideast for religious reasons (all Muslims are required to do this, by the way: a Haj is a requirement for everyone before death). Most of the people I know who most vigorously campaign for anti-gun legislation also accuse Mr Trump of being racist (actually, Muslim isn't a race, but whatever), and I'm of the opinion that a devout Muslim, who is an American, does have a right to bear and keep firearms (but not if he or she loudly proclaims he/she is going to shoot innocents with said firearms).
At this point, "not my circus, not my monkeys." Talk your state legislator. In a few years, maybe I will run...