The reasons war is generally considered bad are that it is expensive, it displaces a lot of people and it leads to the injury or death of many others. War is also very traumatic to those who experience it. Many people, including soldiers, never fully mentally recover from their war experiences. It also leads to the destruction of landmarks and cities.
Organizing, training and deploying military forces is extremely costly as is executing military strategies. Aside from those who fight on the front lines, there are thousands of others who must strategize military action as well as ensure that military and civilian personnel are properly equipped. All of this costs money.
Fighting during war time is often deadly to civilians as well as to soldiers. Many families lose loved ones in times of war. Many injuries are incurred during war as well. Some of the injuries are so severe that people never fully recover. The United States alone has lost more than a million soldiers over the course of all of the wars it has fought. Another 1.5 million have been injured, and almost 40,000 people have gone missing. Those numbers do not include civilian deaths and injuries. Some people are so mentally devastated by their war experiences that they are mentally scarred for the rest of their lives.