Of course, it kills a lot of people.
It wastes natural resources, such as oil, metals, etc.
It requires more warfare or threat of warfare to secure those natural resources for the most powerful armed forces.
It wastes money that could be used to solve problems and help people, both for individual nations and for the world as a whole. Enough money to end poverty, provide health care to everyone, educate everyone, and go a long ways towards solving our environmental problems.
It creates hatred between peoples.
At a time when we face immense problems that threaten civilization, human survival, and even life on Earth, we cannot afford war any longer.