James exhorts believers to 'show no partiality.' The reality of partiality is that we all have it in our relationships. We tend to deny that we have partiality but James did not hide or cover up their issue. Instead he points out that they are showing partiality based on people's wealth (v.2-3). As he did, we should be able to admit we have partiality, too. Moreover, we should know that it produces bad fruits. We have many examples in and out of the bible. It hurts people's heart and distorts the relationships.
The root of partiality is our evil thoughts and judgment based on those thoughts (v.4). Therefore, it is a sin because having partiality means that we are sitting on the God's position and judge people with our thoughts which is evil or not enough to judge others. Also, partiality is against the royal law, 'love your neighbors as yourself' (v.8-9). And this sin is relatively considered as a small sin, yet James compares it to murder and adultery (v.11). It's because no matter which law breaks, we found guilty of breaking all of it (v.10).
Then, what is the remedy of partiality? It is mercy. It is love. Not showing partiality (passive) is not enough. We should show mercy instead of partiality (active). That is the goal of the law, and that is the best way to get rid of partiality in us. Mercy triumphs over judgment! (v.13).