The main point to keep in mind is that there were many, many times MORE people alive during the 20th century than during any other time in history. Therefore, there were more people around to be killed during war. If the comparison is simply raw numbers, without putting those numbers in context, then, yeah, it will seem like the Society has a point. Ditto for famines, disease, etc. What you need to do is look at what percentage of the total global population those numbers represent. When you make that the basis for comparison, suddenly the 20th century numbers aren't so impressive.
And the explosion in population is really staggering--For most of human history, deaths roughly equalled births, which meant the population stayed static. It wasn't until the Industrial Revolution in the late 18th to 19th century that the population started to really grow. By the early 20th century (pre-1914), it was soaring and continued to soar all through the 20th century until the late 1960's.
But this leads to another point you might want to raise with your conductor. It is a well-established demographic fact that when populations (whether fish, birds, or people) are under stress (ie famine, disease, upheaval, violence, etc.), they do not grow, or grow extremely slowly. In fact, the combination of factors (ie famine PLUS disease, or disease PLUS war, etc.) has a depressive effect on populations--they shrink. That's one reason why so many animal species are endangered right now. Yet when you look at the 20th century, you find that human population exploded--exactly the OPPOSITE of what you might expect if we really had been living in the worst of times. Why is that? Because even with 2 world wars and all the social and political unrest, things were getting better. Medical advances meant people WERENT DYING of disease. Agricultural and technological advances meant people WERENT DYING of famine (okay, obviously many did--I mean its relative when looked at globally. For instance, when famine strikes today, often the UN or other nations step in with food supplies, which simply didn't happen 200 or 300 years ago).
The point is (and this is what got me thinking seriously on my way to the fade), what should population demographics look like if things really were as terrible as the Society makes them out to be, especially given that these "terrible times" have been going on for a century now? For one, populations should be nose-diving. And they're not.
This is too long a post, but as final thought, you may find it intersting to check out the UN's Millennium Goals Update . While the picture is not all rosy by any means, it's amazing what has been accomplished. You won't find info like that in the Bible Teach book.