I really can't answer the question. I wanted to know if the person was planning to use it on himself. But, this person seriously needs a wake up call and a better way to deal with anger. It might be worth asking some questions, one you could ask:

Is this person in a green truck worth going to jail for life, or death row for that matter?

All I can say is that a dialogue is needed to get him thinking straight again. What did the person in the green truck do? Cut him off? What if they were rushing to the hospital? What if they had just gotten into an argument with their parents or significant other? What if they are indeed rude all the time? Is it worth killing the person? People change, and by killing the person, would your friend be any better?

You know him better than any of us. But asking questions may get him into a more rational position. If you can't quickly find a solution, or think it won't work, it would be best to contact the authorities.