"Always"?
According to what I've read this shooter was 64 years old. That would make him born in 1952 or '53 and ergo a young and impressionable child when this splattered all over TV, which was considered an electronic babysitter:
His older brother born ten years earlier didn't have that kind of propaganda instrument. He did eventually, but by the time TV got widespread he was old enough to not be so impressionable.
Of course that's just one of the factors I listed. After that commercial, and maybe another one for GI Joe ran, he'd be returned to his program about shooting Indians or bank robbers. Then he'd go to his comic books and read about the invasion of Gongontula and how he was vanquished with a death ray gun. Then when he got older his TV tastes would mature to shows about cops shooting still-more 'bad guys' or Star Trek shooting Klingons, and then older still he could go to movies about the Mafia. And on the way home he might have stopped and played a video game shooting down "enemy ships".
Indoctrination takes time. And it never gives up.
The other half of this is the masculinity aspect. Mass shooters are not out for 'murder' per se -- they're out for power. So again, when they dig themselves into whatever hole of powerlessness they believe themselves to be in, they're going to turn to what their indoctrination has always told them, from the earliest most impressionable age, is the way out.