A lot of places have hunting laws which are aimed at reducing the size of the population of a rapidly propagating species, such as deer. Nature designed deer (among others) to have to deal with natural predators by allowing them to propogate rapidly. With men around, their natural predators have mostly disappeared, so their population can grow, often beyond the environment's ability to feed them. Result: all the deer would starve in winter. So "culling" is implemented, so that by reducing the population, enough food is left for the rest.
The same basic premise can be extended to mankind. Right now, the world's population is doubling every 40 years or so. That means by 2100, the world's population will be somewhere around 40 billion people. Mostly in poor parts of the world.
Can the Earth sustain 40 billion people? And look at the year 2200 - at the same rate of growth, we are talking 240 billion people! Technology (in the form of better farming and processing) and trade (getting the food from where it's produced to where it's needed) will certainly help, but at some point it will be impossible to keep up.
One of the things that the modern world has grown out of is full scale war, which would take care of sizable chunks of the population every once and a while (around 45 million in WWII alone, mostly in Europe). Partly due to things like the UN, easier communications, and the fear of nuclear escalation has mostly kept conflicts down to border clashes and limited engagements over the past 50 years.
Another, even bigger double-edged sword is medicine. Smallpox, malaria, plague, and other diseases which used to kill off large swathes of the world's population every year are now more or less under control, and in some cases even eradicated by the development of vaccines and antibiotics.
Do we want a good war to carve off a big chunk of the population? Do we want a return of unstoppable infectious diseases? Of course not, although it would solve our population problem.
Of course nobody will contemplate culling the human population intentionally. Well, maybe some would consider it, but Hitler, Tojo and Saddam are hardly role models to follow.
The only solution left is to limit the birth rate. Unfortunately, the only countries that have the infrastructure to enforce a neutral growth population program without resorting to barbaric methods are the countries that do not have huge growth rates. The problem areas are the poor countries, where people still believe that having lots of kids is the best way to secure your old age. "Civilized" incentives to restrict family size (for instance a heavy surtax burden if you have more than 2 kids) is only possible where people live structured lives and pay taxes to begin with. Hardly an apt description of the Sudan or poor parts of India.
What's the solution? I would say the first step would be globalization. Only through globalization of the world's economies can the poorer nations develop to the point that they can structure themselves to gain some measure of civilized enforcement power over the population, and economic development which will allow the lower classes jobs and incomes that are different from the scrape-and-gather mode of existance they are used to, and that makes having a lot of children so attractive.
Of course at the same time you would have to make sure that corruption is under control in these countries. It is institutionalized corruption that has kept so many naturally rich countries like Congo and Phillipines from developing properly.
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The only thing you should feel when shooting a terrorist: Recoil.