It's no justification but the Mcyclist was a bit of a knob, he approached the roundabout in the turn left lane, went straight over, then used his horn and gesticulated at the car who was going straight on, from the correct lane and exited into the left lane, none of which was unreasonable. The bike should have just slowed a bit and let the car carry on.
What followed next with the car cutting him up was unreasonable, as was kicking the car, as was the violent reaction that.