Just ask your liberal relative to look at this scenario.
GZ is in his truck going to the store, he sees a stranger loitering around in the rain. GZ watches for a minute, and decides it is no big deal and goes about his business. The next morning a neighbor doesn’t show up for work, can’t be reached and the police enter the home. Several bodies are found, and the house has been ransacked. The first question to GZ would be why didn’t you do more, you were the neighborhood watch guy.
He’s convinced that GZ went looking for a black person to confront. Your hypothetical situation is too close to reality for him to take it seriously.
I’ve tried discussing things with him in the past, but it’s like mud wrestling with a pig. He makes up his own “facts” and refuses to acknowledge any facts. So when he tried to goad me into an argument about this, I knew he wouldn’t be receptive to any facts, and I just gave him my best “you’re a moron” look, and left it at that.