Sorry I'm late, just got home.
The guy may or may not be a Christian we don't know. Him sporting stickers on his bumper claiming he is and acting as you discribed, he is a poor witness to people that don't believe and give the rest of us a bad rap.
You did the more Christian thing by ignoring his bad behavior.
Maybe he was having a real bad day and maybe he is a new Christain and still kinda in his old ways but trying to do be more like Jesus. When you wanted to merge over he forgot how he should respond and lost it. We don't know.
Even on my bad days I don't act like that.