Hostility seems to be a hallmark of this church age in a similar way that road-rage is to the world. It is alright for us to be righteously indignant as long as we do not sin. There is a place for righteous indignation, but God does not permit much anger because it is difficult not to sin when angry. That kind of anger is a "mark of the beast."
Frequently, hostility is simply a denial of reality. People do not have tempers born in them; angry tempers begin to be created in childhood. Parents allow tempers to burst forth, and each time it happens, it becomes easier—and the next time and the next and on and on until it is ingrained in the personality.
Anger is nothing more than a passionate response to some sort of stimuli, and it is almost always a self-centered response. It usually begins when we believe that what should or should not have happened either did or did not, and conflict arises. We can believe, either strongly or weakly, it should or should not have happened. Therefore, anger can be either strong or weak or anywhere in between.
The reality is this: What happened happened. How will anger help the problem? Satan believes that it does because he wants to control, to win, to compete, to devour, to get the upper hand, to triumph. Do we really need the anger to drive us to manipulate or to punish? Why not just start working on a solution without the anger, knowing full well that the anger will likely create sin and cause additional damage to the relationship? In a way, it is all very logical, but our feelings get in the way.
Proverbs 14:12 says, "There is a way that seems right to a man, but its end is the way of death." The first clause can be paraphrased, "There is a way that man thinks things should be." This is where conflict arises: Two people see things differently. The question is, then, who is to say that it should be the way we see it?
Things happen because laws are broken, and whatever we sow we reap. Sometimes we get caught in other peoples' ignorance and stupidity. This is a fact of everybody's life, even to God in the flesh. He got caught in the ignorance and stupidity of His fellow Israelites in Judea, and it cost Him His life—yet He did not get angry. What an example! What an example of control. "Let this mind be in you, which was also in Christ Jesus."
How far did He go to make peace? To the death. Even when the other person was totally, absolutely, completely wrong, He did not go to war against him.
The problem with anger arises when we turn our feelings and drives to set things right, as we see them, into absolute necessities. We feel it must be our way, but the reality is that others have the same rights from God that we have. Everyone has free moral agency. Anger arises because of the way we judge things: We apply the standard that we hold as being the right one.