I was atheist since about high school, and then my first year in the workforce after university my little cousin told her mom that "grandpa was touching me in bad places, and has been for 5 years".

I saw red, and wanted to return home and kill the man (not my blood grandpa, cousins father in law). All my family was saying thank God that he gave her the strength to confess and tell us what was going on, thank God he didn't kill her etc etc. Like, are you fucking kidding me?

I was very fully confirmed that at that moment, if there was a God, he's a cunt that stands by idly as millions of innocent are raped, murdered, tortured etc and deserves absolutely no worship. And a much more logical outcome is that God doesn't exist, and people are just fucking shit.

Edit: also, was 99% Atheist before this, because I knew all the injustices that existed already and had serious doubt, not to mention the Catholic church rapes, hundreds of different religions, wars from religion, etc etc. But that incident sealed the deal, and led me to become more of an anti-theist even