Do You Have an 'Ax to Grind?'
War has been a part of human culture for a long time, and weapons are even older, so there are many expressions about weapons that are still used today.
If a person has a problem with someone, you can say they have "an ax to grind."
So if your brother is always rude to your friend you might say, "What's your problem with Fred? It seems like you have an ax to grind with him."
A person can also be said to have an ax to grind if they have a hidden or selfish reason for doing something.
So if you noticed that your brother is always very competitive with your friend, you could say, "Why is it so important to you to beat Fred at everything? It really seems like you have an ax to grind!"
You can call an unpredictable and dangerous person or thing a "loose cannon." Cannons are large, heavy guns that were often on wheels when used on ships. This meant that if a cannon was loose — meaning not held in place — it could move unpredictably and cause damage.
So your brother might say, "Every time Fred comes to our house he breaks something. He's a loose cannon!"
If two or more people agree to stop fighting and become friends, you can say they have "buried the hatchet." This expression comes from a Native American tradition, where groups that were fighting with each other would bury their weapons when they agreed to make peace.
So you might say to your brother, "Well, Fred's my best friend, so you need to talk to him and bury the hatchet."
