James begins this chapter telling those who trust in their riches and oppress the just what their end will be. We know that no man is justified before God except through Jesus Christ. So since James says these people have condemned and killed the just, we can assume he is talking about those God has justified----Christians.
We see it happening all over the world, even in our own United States now, Christians being taken into court and condemned for standing true to their faith and true to the Word of God. We see Christians in other nations being put to death daily because they stand on the truth of God's Word and will not recant.
We can be discouraged or we can rejoice and take heart that God has seen fit to tell us the truth of what will happen no matter how unpleasant it may be. He knows all things and is in control of all things. He can either sustain us through the storm or He will take us home to be with Him which is far better.
When someone says, "Look how terrible things are in the world! Where is God in all of this!" All we have to do is point to passages like this and others that predict hard times and say, "God told us it would be like this. It's no surprise to Him. It just proves He exists and that He loves us enough to tell us the truth of what will happen in a fallen world."
Those whom James is talking to here will have their day of judgment before God. This is a prediction of that. God even cares enough to tell the persecutors what their demise will be if they continue on the path of destruction they are on.