There's one person responsible for this, the shooter. Period.
I really can't see how your original post is at all helpful.
But, there are things that can be done.
1. Raise the age to acquire military style semi-auto rifles (I wouldn't care if they were banned, btw) This won't stop acts of violence, but would likely reduce the body count. Many of these acts are carried out by men under the age of 25.
2. Monitor online postings of extremist groups. Apparently it was known that he had posted a "manifesto" online. There were probably warning signs.
3. Mental health research. A shockingly high percentage of these acts happen by people either starting or stopping SSRI medications.