Be angry, and yet do not sin; do not let the sun go down on your anger, and do not give the devil an opportunity. (Ephesians 4:26-27 NAS)
All of God's children have what I would call a built-in "immorality and injustice detector." When we see what is against God, it is instinctive to be angry at it. That's a good thing. What happens after that anger can be a serious problem.
For instance, someone may become angry at abortion. They find the civil law will not stop it, and protests against abortion clinics don't do any good. All their anger gets turned into frustration and bitterness and possibly despair. If allowed to continue, that bitterness will turn to hatred. As Jesus taught, hatred is spiritual murder. Sadly, that hatred can even turn into the act of actual murder of abortionists.
To the person involved, it all seems very righteous. It begins with a justifiable anger against immoral and unjust acts, so the end result seems to be justified as well. It is, "christ-like" to the person involved and they can eventually justify the most horrid acts as a result.
It is no wonder then that Christian churches and other religions can fall under the attack of this "christ-like" spirit. Secular social movements and civil governments do the same thing, of course, so it is not limited to religion. It is common among mankind's institutions.
This is what Paul is warning about in Ephesians. The justifiable anger becomes a foothold for the devil and the adversary then fans the flames of righteous indignation until it becomes a roaring bonfire of hatred. It is a difficult thing to detect because it seems so right to be angry. But the anger must be directed somewhere constructive or it must be released through love and forgiveness. Otherwise the devil can use it to his advantage.