When I was younger, I was always that boy with a very short fuse. It didn't take much to get to me, and I had to pay the price for my short temper many times and sometimes in very dear ways. Nowadays, I don't struggle with it much, but I still lose my temper every now and then.
Is it a sin to be short-tempered? Well, not necessarily, but it is really dangerous and people with short tempers can find themselves falling easily into many sins. Anger is an emotion that was created by God just like any other emotion, and He desires that we put it into good use.
There are two situations where anger becomes an issue:
When anger is driven by selfishness
The very first time a person was recorded to have lost his temper was when Cain was driven by anger to kill Abel. Why was Cain angry? The root of Cain's anger was a bitterness towards God and Abel that was fuelled by a selfish desire. Genesis 4:5-6 says, "but for Cain and his offering he had no regard. So Cain was very angry, and his face fell. The Lord said to Cain, 'Why are you angry, and why has your face fallen?'"
When we start putting ourselves before others, we can easily lose control of our emotions when people don't take your needs, desires and preferences into consideration. This causes us to act selfishly and selfish desires always lead to sinful actions. Psalm 4:4 says, "Be angry, and do not sin; ponder in your own hearts on your beds, and be silent." Short-tempered behaviour can be a sure sign of selfish motive.
When emotions control us instead of the other way around
Many of our feelings and desires come from God, but even the most well-meaning desires can bring problems when they start controlling our will and actions instead of us controlling them. Martha must have meant well when she lost it and started berating Jesus and Mary for leaving her to work in the kitchen alone, but she had lost herself to a desire to perform.
Galatians 5:23 tells us that one of the facets of the fruit of the Spirit is self-control. Anger, when channeled and controlled can lead to righteous action. When we're angry at sin, injustice, trials and our well-controlled will drives us to trust God more and do our part, anger becomes a tool to do things right. Short-temperedness really is just a manifestation that we don't control our anger, but that the opposite is true.
All emotions can be used to point us to God - happiness, sadness and even anger. But we must always remember to submit our emotions to Christ never allowing them to overcome the reality of the cross in our lives. When we are not in control, we can still remain in control of our emotions because we know God is in control.