This isn't the "what radicalized you" line of questioning. I always read those as the last straw. I'm more interested in your first steps.
I know everyone's journey is different.
Mine was realizing that despite being a complete asshole, I give a fuck about everyone.
First step away from the status quo was the Snowden NSA leaks, which led me to two things: anti-war and other criticisms of the US from Wikileaks, Intercept, etc.; and to free software (FOSS) where i picked up other leftist ideas. Took me a while to consider myself any kind of socialist though, I thought I was a centrist libertarian(?) for a while.