Donate to a human rights organization and/or volunteer in one. I'd say that's the best you can do. And remember that however horrible it sounds don't lose sleep over the suffering of people you don't know. There's a high risk it'll just consume you.

Our tax dollars pay for the weapons used on children. The outrage for Western support should be unanimous and visceral

The massacres have been occuring through all of human history, the world does not see any years without wars and atrocities.

The question I'd ask people is how they suddenly notice when this is happening.

If you're struggling to ignore it and carry on with your life, think back to the point where you were blissfully ignorant of the suffering in the world and return to that state.

As a single person, most of it comes down to what you support, avoid consuming things that rely on cruelty and slavery to be produced, study to be more aware of what is actually happening, and don't panic, there's very little a person alone can do.

Joining groups with similar minds can help you make more, but groups can be very hard, people have different views on things, and radicalism can make some good willed people do pretty terrible things.

We need to understand and dismantle the systems that produce them.

Atheism and Feminism. Both have shown to lead to more pacifistic societies.

Make the world around you better. Locally. There is so much going on in the world and we get news about all the wars. You as a citizen can't fight every one of them. But you can be nice to people around you, volunteer in your local community. Make the world a better place where you are. Help people that get targeted in your country. Fight right-wing parties where you live.

Atheism. Most of the atrocities in the world are driven by religion. As the saying goes "good people will do good, bad people will do bad. When good people do bad, there's probably a religion involved"

Am I just supposed to sit there and watch as all sort of injustices are taking place everywhere?

For most of them, yes. Humans didn't evolve knowing every horrible thing going on around the world, and you individually can't possibly do something to stop all of them. Pick a few causes that are most important to you and ignore the rest, that's the only way to stay sane.

Considering the political machine is responsible for most of those, the only way to enact a large enough change is to keep pushing your disapproval online and to your locally-elected official(s) with reasonable arguments why. If the public opinion turn on those who currently hold a seat they'll have to do something about it or they'll lose their power, and that's the only thing they ultimately care about.

Do what you can to challenge fear, anger, and hate (a.k.a. critical thinking).

They are the tools of the power hungry and wealth hoarders. Rage bait news articles, religious extremism, labeling the "others" (race, religion, sex, national origin, etc.) are how those tools are applied. They are applied to get voters to vote against their own self-interest or even conduct extreme acts like suicide bombings.

Feels grim stated like that as the hate machines and their incentives are already in place and controlled by the worst of us.

Using these tools is also an easy trap to fall into unknowingly. I'm passionate about privacy enabling technologies. I've slipped into fear mongering when on my soap box without realizing it until later upon reflection.

Nobody can speak for victims or suspects except for themselves, but when people talk about massacres like that, people almost always cite one of two reasons, catharsis (think the ones talked about on the psychology programs) and philosophy (think of your stereotypical hate criminal in this case).

I refuse to be a judge on whether these are true, but in the case of the first one, if true, just be there for people. Perhaps you believe in the butterfly effect, in which case choosing specifically downtrodden people as friends is akin to saving hundreds of lives. Check on your neighbors if you must, give to the fatherless and the unlucky, provide compliments, hear the unheard, forgive the unforgiven, be a guardian angel, etc. It is true that one thing that always strikes me when this kind of shooter happens is how clique-ish and honor-obsessed their communities are. I have friends in New York and they often talk about how Upstate has very few shooters compared to Downstate and many cite that Upstaters act very quasi-Canadian while NYC is the city from Judge Dredd by comparison.

On the other hand, hypothetically, as for the other kind of mass killer, can you really help them? If it's as simple as that, I would liken them to people innovatively taking their liberties. Be peaceful but challenge their ideas and keep notes on who is who that can be added to a collection.

Vote for gun control if you're American, its worked in Australia and New Zealand. Or vote for any party that advocates for the intake of refugees. Beyond that build an iron man suit fly over and lay down the law :p

Education, education, education. Separation of state and church. Fair payment.

Ensure the people who would be massacred have the means to defend themselves.

A person’s self interest is more reliable than rules and treaties. A weapon in a person’s hands makes it impossible to hurt that person without yourself being hurt.

This is why after thousands of years of bloodshed and injustice, some very concerned thinkers decided it is a human right to be armed.