Most arms dealers care only about the profits they make from their sales. They do not tend to take sides and therefore have no conscience about who buys their arms and how they will be used. Even if they do sell arms to what they believe is a just cause, the arms can easily fall into enemy hands in the case of defeat.
Guns are not automatically violent or bad because they kill. Guns can serve plenty of purposes without killing. The obvious exceptions are hunting and sport shooting, but bigger weaponry can be useful too. When nations have large armies and arsenals, it discourages military action from other countries. The weapons don't have to be fired to be useful.
Arms dealers are not any more immoral than anyone else. Arms dealers can be more moral in one sense that they help people and countries defend themselves against aggressors. Most arms are never really used. They are primarily deterrents. It helps defend the little guy. Of course arms dealers do not get involved in actual killings.
Arms dealers are not automatically immoral Yes, some arms dealers can be unscrupulous; selling dangerous weapons to evil people. However, most arms dealers provide an important function in the market for weapons. Weapons enable countries to secure their borders, protecting their sovereignty against enemy aggression. Without arms dealers, many countries would not get access to the defensive weapons they need.