In the 1900s, the leading causes of death were tuberculosis, cardiovascular disease, and influenza (the flu). Today, the leading causes of death are heart disease, cancer, and stroke. We have very few deaths due to tuberculosis in today's world because of better hygiene from people and doctor's, as well as better detection and treatments. Cardiovascular disease was a big killer in the 1900's, and it is still a big killer today. It is the number one cause of death in today's times due to our poor diets and lack of exercise. The flu is no longer a huge cause of death due to vaccinations, better hygiene, and better treatments.
Also, wars and genocide (two atom bombs), genocides in Africa ad Europe, Bangladesh, and partition of the Indian subcontinent. If we live this century well, we can avoid some of these avoidable causes of deaths. Finally, if someone counts the deaths due to inner-city violence and drug related violence --around the world-- that number over the century might exceed the number of people killed in many wars.