I appreciate what you’re saying, and how you feel, because I bet most people have been in a similar situation where they wished they had spoken up when they felt they should have — I know I have, and yes I felt awful about myself afterwards.
That brings me to my question: isn’t this article just as much as about how you felt about yourself as well as how you felt about the asshole you’re writing about? I’m detecting this article isn’t really about the guy who was an asshole (because the world is full of them), it’s really about you feeling annoyed with yourself.
The world is full of mirrors. Perhaps this guy acted as a mirror in some way, showing you not only what he lacked, but what you were also lacking (not necessarily that you’re an entitled asshole like he was, but showing you that you lacked the courage to stand up to him when it really mattered, so that his victims could gain some justice from being mistreated), therefore now you feel annoyed with yourself? You said he acted out of powerlessness, maybe you did too because his feeling of powerlessness brought that out in you, hence the delayed response and afterwards, this article.
If that’s the case, then I strongly suggest you stop beating yourself up, and learn from the experience. Make a commitment to stand up for those who can’t, when and where it really matters.