Firstly, Maniac, you know what maple's answer is going to be.
Secondly, he charged the witness stand, apparently violently, with a deadly weapon (a pen to the throat is very much deadly), and moreover, it was a witness who betrayed him in testifying against him. I don't condone the shooting necessarily, but some course of action had to be taken, and this is the course of action they have in place for such events.
Thirdly, no, life is not that cheap in the US, but had he gotten to the witness stand, we'd have two deaths instead of one, or at least a witness seriously hurt. They saved a witness here, and while the end result is certainly not what it should be, let's not forget that.