Was it a one-time sudden realization, a nagging feeling you couldn't get rid of, or a back-and-forth process?
For me, it was 5 or so years of back-and-forth waffling between vegan and "reducitarian/VAH" before a very strong dose of magic mushrooms a few years ago gave me a stern kick in the ass about my own ethics and hypocrisy, which is how I finally stayed vegan. The same trip was the beginning of my political radicalization left as well.
Mine was pescetarian for a few years, then watched I think Paul Mccartneys Doc on Animals (which was the first animal doc I've ever seen) and then watched Dairy is Scary? I think it was an hour later and that solidified it for me, I couldn't ever do it again and it fit in perfectly with my leftist philosophy,