Overpopulation in terms of running out of space is definitely overplayed. But it's a direct multiplier for our effect on the environment. If the world's population were cut in half, we'd emit about half as much pollution, half as much greenhouse gas, half the land used for farming would be left wild, etc.
Not necessarily...carbon emissions per capita is not constant around the globe. Energy use is skewed towards modern countries, i.e United States. If you wiped out the 300 million people in the US, you would save more on carbon emissions than wiping out 300 million in China.
