Killing 20 million people - mostly in context of two world wars and civil war between them - cannot possibly increase standards of living of the rest. ... This is Malthusianism taken to ridiculous levels.

Just an FYI: This position you're arguing against is exactly what Gregory Clark argues in "A Farewell to Alms [sic]", minus the bit about destruction of capital. (See the links to reviews that summarize it.)

While I don't agree with his thesis, it's a serious attempt to show that massive die-offs (like through plagues) increased per-capita income. He shows records in England of the prices for capital (wealth-producing) goods like cattle going down as the population went down, thus making it easier for someone to make a living. This was, of course, before the 18th-century, and in that time the world actually did match Malthus's theories because of the lack of technological breakthroughs necessary to sustain higher populations.