This is addressed to white people specifically. I hope that's ok.
Caveat or, mea culpa - whatever the term is; full disclosure - I am myself white people, though I prefer the term "personage."
White people, at some point today if you haven't already, you will see a viral social media post, with words to the effect: how ironic is it that all it takes is one white woman to get publicly killed in a horrifying video clip to get tons of white people outraged and speaking up?
There are a lot of reasons for posts like this, but let's focus on the end result. The aspect that matters: a lot of people would like you to shut your mouth. Racists especially, white supremacists particularly, would like you white people to shut up. They would like any white people arriving late to the fight to feel ashamed and fake, and shut their mouths in embarrassment. They would like you to question the basis of your outrage, and be unable to come up with a flattering answer. They want you to go back to being the average white person who finds all this so unpleasant and distasteful, and acts like absenting their voice is some kind of high road. They want you to slink back into silence, and let people with strong convictions hold the field unopposed. They're confident they can beat the activists, if bystanders keep out of it.
Well, don't shut up. If seeing that was what it took to open your eyes and open your mouths, don't close your eyes now that they've been opened. Don't shut your mouths. There have been a lot of horrific videos in recent years. There will be more to come. Your voice is powerful, especially in its silence if you let it be silent. Whatever it took for you to find your voice, to locate your outrage and to direct it at those who have contempt - not just for people who aren't like you! But for every human life that doesn't embody their twisted, lost, hateful ideology - whatever it took to wake that up, don't let your outrage go back to sleep now. When the battle is pitched and you are desperately needed, don't quit the fight just because you arrived late.
If at worst, you're forced to feel a pang of guilt for not having spoken out sooner, embrace that pang. Maybe you're one of the people like me, who somehow can't believe people are that bad. Maybe you've been able to shunt away how bad things have been getting, because it didn't cross over into your own life. Maybe you couldn't believe it because nothing had yet brought it home to you. Maybe you were able to rationalize away other attacks, incidents: deaths. "I wasn't there." "Maybe something else happened, to make it make sense?" No, you weren't there. It's been a privilege. Be there now.
Don't shut up. Engage the people who refuse to condemn what is patently and thoroughly evil. Ask them what on earth is wrong with them.
And yes, be aware that there's going to be a bunch of folks who make scathing ironic comments about how long it took you. They've had a long, disgusting, disheartening slog, and their patience is far from fresh. You'll find out a little bit about what that's like, so long as you don't turn over, close your eyes again, shut your mouth now.
Don't give evil the silence it needs to grow unopposed. Don't let your past silence shame your outrage now. Raise your voice, and keep raising it. Do you hate racism?
They will hate that.