Okay, let's just say, for the sake of argument, it's completely legal to split lanes.
Biker was splitting lanes. Yay, you're totally legal dude.
Mean, douche driver cuts off biker. Hey, that's a foul! Driver in the wrong.
So, biker kicks the car. Appropriate response? Well, to me, no, it's not. Seems a bit too much, really. Driver cutting off biker is not as egregious (nice word, BTW) as biker kicking the driver's car. Rather, it appears that biker has escalated the confrontation by using a response that is not appropriate to the insult.
How about this: guy is riding a skateboard down the sidewalk. Another guy intentionally steps out in front of him, cutting him off. Skater wobbles, almost crashes, but doesn't. Skater stops and, in his anger, slaps the guy in the face who cut him off. Guy then body slams skater, breaking his back.
Who is in the wrong in that situation?