Myositis. Not usually. There has to be more going on to succumb.
Depends on type. Polymyositis / dermatomyositis kill only if they produce extreme weakness that compromises breathing or results in skin damage, or if the heart muscle is involved or (as often happens) the disease also causes fibrosis of the lungs. Other forms of myositis are even less likely to kill. Your physician can tell you what your diagnosis means to you.