Everything that happens in life is down to intersections of causality.
A person crosses a street. A driver drives down that same street. Depending on the actions of each, those two actions - those two lines - can cross in different ways. In the worst way, they cross in a literal impact; but that is rare, and only happens so often due to the sheer number of cars on roads and people on pavements.
A couple meet due to actions and consequences taken all the way up to that point - their relationship forms as based on those lines, how they intersect, what they intersected with beforehand. Decisions made in the past, the results of circumstances and events, all roll forward to change the present and the future.
Everything, from important to inconsequential, happens this way.
When things happen that hurt us, that scare us, that make us angry - the temptation is to make the event personal. To make it all about us. It is also perhaps tempting to chalk it up to pure chance.
A company makes weapons, and they want somewhere to sell them. A government who are contractually bound to underwrite those sales finds a buyer. That buyer uses those weapons to murder innocents, and also sells them to those who would do the same. A man who receives weapons teaches his followers how to die for the cause the weapons are for,. A person decides to hurt other people because of a cause he has been taught is more just and valid than any other. People are hurt.
Each of these lines aside from the last features an abandonment of morality, a suspension of the concept of wrong and right. Each injury and each death caused at the far end of these lines begins with the decision to do something wrong.
It wasn't just those that deliver the final blow, though there will be those that want to get us angry, to make us hate, and will hold them up as the sole reason why this happened. These people decided to hate before the attack even happened. It's a tool, now - the line of the attacker has crossed theirs in a way that they will decide to use to push their own agenda.
How can we have stopped it?
We can't - but we can draw new lines.
We can't necessarily control the company making the weapons, but we can pick the government that rules us. That government can, then, make different decisions. It can draw different lines. Intersections that previously existed will fade away. We can choose to hate less and learn more, which will draw lines in how we are observed. We can refuse to let our decisions bring into power those immediate vultures, those who seek to turn our initial hurt into a scurrying for safety under the shadow of tyranny, or a burning rage against everything that isn't us.
That's a choice WE have to make, because that's all there is - the choices we make, and the ways in which they can change the world later. The lines we draw, and how the lines cross others.
We need to make sure we draw the best lines we can.