Jebbus, man - that's one of the most randomly malicious things I've ever heard of. Like everyone else, I'm glad it didn't hit YOU - that could have easily been fatal in the wrong spot!
The douchebag deserves the beatdown of his/her life!!
Oh - and you're right about the effective impact speed/kinetic energy being additive (minus decel. due to air resistance). 'Had dynamics' and understanding vector mechanics are obviously two different things.
