"can you blame them for reacting so strongly?"
Oh hell no. You should be able to shoot anyone who may cause you to brake hard, use evasive driving techniques or exercise self-restraint at any time during your lifetime.
If someone serves you lumpy gravy, cap their ass. They endangered you by causing your blood pressure to rise.
Peace up homey. You got it going on. No dissing you bro. Slap the ho silly.
Same mind set as your's but it's said in a different manner.
You're making it sound as if the internal threat/panic of being hit by a foreign object while driving is an everyday occurrence.
The very fact that the driver reacted so strongly suggests he felt threatened and attacked in a way he's never experienced before.
And unless/until your adrenaline has been activated to that extent, it's hard to pass judgment.