The society gets stupider. Everyone thinks they can solve any conflict with a gun.
And a lot of people think that they are under no obligation to minimize the causes of conflict with others. This is the entitlement mentality at work. Evidently, people are entitled to have pets and to impose the burden of those pets on their neighbors.
Convincing people that you are insane is sometimes a way to reduce conflict with them. Apparently this shooter was not successful in convincing his neighbors that he was murderously incapable of peacefully resolving this issue. There is no "win-win" for any of them.
There is now an opportunity for some other senile gun-owner to clip a copy of this article and anonymously deposit it in his neighbor's mailbox.
There is a lesson in this for all of us. When some people get to be 75, they just may value some things above life itself; that is, above the value of YOUR life.