He stopped the killer from killing the boy he was targeting
Conjecture. He was also under fire from law enforcement. You have no idea if he would have hit or not. You say he "stopped the killer" because that supposition best helps your argument.
The fact that it was an off-duty police office is irrelevant btw
It's absolutely relevant since few people argue against having armed law enforcement officers and make the distinction between police training and someone buying a gun because an ad told them it'd get their "man card" back. But you already knew this which is why you're scrambling to discredit the relevance.
If the same off-duty police officer had been at the Sandy Brook school last Friday (perhaps because he had a kid in the school), he would not have been able to stop the shooting because he would not have legally been able to carry a weapon with him on the school (or even near the school).
He would have been legally allowed to carry it if he had a carry permit issued by the state and that state requires background checks...
(B) Subparagraph (A) does not apply to the possession of a firearm—
(i) on private property not part of school grounds;
(ii) if the individual possessing the firearm is licensed to do so by the State in which the school zone is located or a political subdivision of the State, and the law of the State or political subdivision requires that, before an individual obtains such a license, the law enforcement authorities of the State or political subdivision verify that the individual is qualified under law to receive the license;
...which fits the CT carry permit requirements.
So... great job passing laws that get kids killed, I guess?
I bet that sounded more clever in your head when you thought you were right.
I'm just making the point that you will insist that every time someone pulls out a gun they must have stopped a mass shooting but you can't point to a single bona fide FBI-defined "mass shooting" that was actually stopped by an armed civilian. It's not for lack of trying -- people have intervened and died for it without stopping the shooter. As I said, it's as though you must get magical bullet immunity after it becomes a mass shooting.