What part of our society is ultimately responsible for the shooting at Stoneman Douglas High School in Parkland, FL?

The part named Nikolas Cruz

The part that has no self-discipline and courage to work through adversity instead of giving into their feeling for revenge of whatever they believed wronged them. Make no mistake about it some people are wronged, but that’s no excuse to grab an AR15 and shoot up people. MHO

the criminal who did the shooting.

I feel it ultimately the shooter’s responsibility

I'd have to place all the blame on Law Enforcement. They knew he was a "Head Case," several years before that.

All parts of society are responsible.
1. The Church stopped leading by example
2. Welfare:
A. It split families apart
B. Made Mother's dependent on the free medical and child care
C. Children are not taught empathy for others.
3.

In the 20th century every nation that disarmed their citizens went on to kill their citizens in massive numbers. WW2 era Germany 12 million at least, Soviet Union 40 to 60 million, China 40 million, Cambodia at least a million. That can't happen here in the USA I am sure the citizens of those nations thought the same thing. When they take your guns they take your freedom, they have stolen the nation and all the principles we were founded upon. Descent will be met with imprisonment, impoverishment and or death. It is a "heart" problem not a "gun" problem.

Historically, it isn't much easier or harder to legally obtain a firearm now than it was 30, 40, or 50 years ago, yet the United States didn't have mass shootings as frequently as we have them in the past 10-20 years. There are a whole host of things that most likely are contributing to the increase in mass shootings, but the availability of firearms isn't one of them.
I think responsibility at all levels, or lack thereof, is contributing to so many of our problems, not just mass shootings, which is one of the most horrific symptoms of the moral direction our country is headed. The issue of mass shootings also shows our inability to correctly identify the root cause of any given situation, which makes it awfully hard to address the situation at hand.

I think that society wants to blame an inanimate object. Disarming the masses leaves the wolves to slaughter the sheep. Mental health is a huge reason these events are taking place . Those high school students admitted and defended the fact that they had bullied the shooter in Florida. From the school shootings in Columbine, the shooters have been bullied outcasts that finally have had enough. Multiple epic failures on behalf of Law Enforcement, Mental Health Services, and other students as it is easier not to get involved, than to stand up for someone who is being bullied and tormented. The other problem is Social media as there are those who want their 15 minutes of fame, and want to out do the previous deadly rampage. Gun Control will not end multiple victims attacks, we all as Veterans know that a firearm is not the only thing that can create multiple deaths incidents. And sadly it is just going to be a way of life. Those who want to kill, will kill with whatever means available. I by no means whatsoever condone these horrific acts. But I do believe everything these kids are exposed to is violence, in television, games, the internet, and no one takes any responsibility for their actions. Let’s be honest, the color of the coat of a Kardashian’s kid, makes more news than the daily slaughter of kids in say Chicago. No responsibility

IMO most of the responsibility falls on the school district's lax to non-existent reporting of criminal behavior to local law enforcement, local law enforcement not arresting students for criminal behavior all for sake of additional funding for 'improving' their crime statistics.
Then there is the mental health contractors that decided not to commit the shooter for involuntary assessment and evaluation when the school finally begged them to.
And some of the students all but admitted they ostracized him and picked on the shooter.
And if law enforcement visits a house 30+ times, and the individual was constantly a problem with fighting and threatening people at the school the school and law enforcement fell down on the job. Hell, even the FBI had a file on him.
And then the Sheriff's office absolutely blew the response