Honestly consider which kind of antidote would work best, for that specific kind of poisonous-meaning.
Just a couple of examples, to "prime the pump", as it were..
IF the poison were white-supremacist evil, THEN the antidote might be that guy .. Daryl is his name, perhaps, who deliberately makes friends with KKK people, & gets them to honestly change.
( if there ever was an example of what the ultra-woke socialist felon Yehoshua "Jesus" benJoseph meant, in his relationships with people, this guy is demonstrating it. : )
IF the poison is the psychopathic machiavellianism of for-profit-corporations, THEN the antidote might be that investigative journalism exists, & deliberately reading "The Elements of Journalism" ( 4th edition, now, I think ), & helping to get other citizens-of-our-world competent in understanding journalism, & how it is the immune-system our world needs, right now.
IF the poison is child-abuse .. THEN .. this one's difficult: it hurts too deep, doesn't it? .. then .. simultaneously protection for children ( like that operation which liberated a whole bunch of girls from a trafficking-ring, the other year, or like help-lines for children, so they've got a lifeline to call, or like organizations/people who systematically make-accountable the perpetrators, reducing the number of them who are out ), those ( individual & institutional ) who force-the-perpetrators into visibility, thus "removing their accommodation", to be .. diplomatic .. about it, or lobbying for better legislation, or making-certain that therapy is both available and successful for survivors of abuse ( some "charities" just purport to do good, & obliterate opportunity while consuming resources padding their administration's ass by design, & torpedoing those scum would improve our world, markedly )
Anyways, you need to discern which antidote you need to be applying, & you need to understand that some lives get caught in stuff sooo horrific that even witnessing it can mangle a life..
That is the actual current condition of our world, our ocean-of-lives.
To alter our world, our world's ocean-of-lives, we need to deliberately turn the tide, and that means forcing the tide to go against the way that DarkHexad, the 6 dimensions of human evil that I've found so-far, wants:
Countering ANY of these dimensions from remaining in authority on our world is good, countering the entanglement network of multiple-dimensions, together, is better, but!..
Remember Stephen R. Covey's principle!
IF your Circle of Concern is too big, THEN it is draining you, making you helpless!
IF you deliberately & strategically reduce your Circle of Concern, .. THEN .. your Circle of Influence will, as you recover, grow!
And you have your balance to earn: my balance isn't relevant to your condition, neither is your balance relevant to how I'm managing, etc.
Each is independent of others, & the whole "I'm this-way therefore you ought be, too" herd-bullying has no actual validity.
I hope that you find stable balance, & I hope that someday you become able to directly & significantly counter whatever harm/evil it is you witnessed.
This is a good question, though. Seeing horrible things on the internet can affect you. Anywhere from getting wigged out to full out trauma is possible.
Personally, if I come across something like that, it’s a good cue to take a quick break from screens. Go for a walk. Eat if you’re hungry. Drink if you’re thirsty. Spend quality time with your pets or plants if you have any. Pay attention to how you’re feeling and do what might be necessary to make you feel good and healthy. In that state, it’s easier to bring yourself back to the here and now. Look around you and appreciate your life. Seeing how real and raw those things are can give a person perspective, but if you overexpose yourself or ruminate on them, it can fuck you up a little bit. Even if you’re “desensitized.”
If you’re lucky enough where things like that aren’t a regular part of your life (as it is for most of us here), use it as a lesson in not just taking care of yourself, but also of others and society as a whole. There are fucked up people in the world, yes. Sometimes the bad guys win and terrible or disturbing things happen, yes. It’s best to maintain pride in knowing you’re not trying to contribute to that problem, in whatever little way you can.
And if you can’t seem to shake it, get some therapy. Not just for that one thing, but, it may be likely you’re slipping into rumination or worldviews that make it difficult to handle depression. Therapy is great, for real. Shop around and find a therapist you like.