Sadly I don't know any more than the rest of us on this one, there's no trove of documents to read through or video to sift. It just stands to reason if you hate somebody enough to publicly execute them, you've probably spend some time watching them in order to build up that hate.

I don't think it's controversial to say that many people on the left repeat attacks they've heard and only very rarely come up with their own. If the killer watched Kirk in action, he almost certainly came across people making the same exact arguments he would make. He saw how Kirk would respond to those arguments and (presumably here) couldn't come up with a satisfactory response. Publicly humiliating Kirk by proving him wrong in front of huge crowd would earn Robinson endless praise, but Robinson was very smart -- smart enough to know he couldn't do that.