James is writing to believers, so why does he accuse his readers of committing murder (James 4:2)?
He is most likely not speaking of physical murder, though many times envoy is a motive for taking someone else's life. But there is a kind of spiritual murder. Envy leads to resentment and results in a focus on the wealthy person's faults.
We comfort ourselves by saying money cannot buy peace, or wealthy people are miserable, or we pronounce any other generalization that makes us feel comforted that we do not possess as much wealth as we would like to have.
James makes it clear that possessions are not to be our focus. God may give us jobs that produce much wealth, or he may give us jobs that do not do much more than put food on the table. In all cases we are to be thankful and use our wealth for God's glory, not for our own.
