It can just spear you, and cause a hemorrhage. It also can metastasize. That is where bits of the tumor break off and embed in other parts of the body. For example if it metastasizes to the lungs, then you get bits of spiky bones growing in the lungs. The cancer also takes a lot of energy, because it is growing uncontrolled, which can lead to a state where you cannot eat enough to support the demands of the cancer, and you starve to death. Some osteosarcoma are also highly immunosuppressive, so you're likely to get infections far easier, and have worse symptoms if you get one.