It is hard to control the anger emotion because it is instinctive emotion. Anger is directly related to threat and fear. Threat of physical harm or mental harm or emotional harm. Its aim could be to save us from danger.
A stressful situation may create anxiety which can convert into anger. Emotions can be reaction instead of response. Whether the anger emotion is hard to control it depends on situation to situation. It is observed that fear is the hardest. Controlling anger emotion needs a lot of practice and in case of fear or threat it becomes tough to control the anger emotion.