Yep. Other than have strict security protocols in every single building, there's not a lot you can do. And we, rightly, don't want to live in a society like that.
Tragedies like this are inevitable and happen (more or less) everywhere. Doesn't make them any less despicable and doesn't make it any less awful if the authorities should have intervened with this guy sooner.
Yeah, but if that person is a powerful lunatic trying to disrupt politics in your country for his own gain, then you might want at least to keep an eye on it.
Is he, literally, a "Nazi"? No, he's not literally a Nazi.
Did he proudly do a salute with Nazi / fascist connotations twice? Yes, clearly.
So I think that him not personally cranking up the gas chambers isn't really the issue here.