Too Much People!
Overpopulation is definitely a problem, but why? Earth does not have enough resources for the 7 billion people it shelters!
Why is overpopulation bad?
Overpopulation is bad because, Earth doesn't even have enough food to feed all of the people there are now, and its only going to get more populated! Pretty soon we will run out of room, and fossil fuels. Fossil fuels are our main source of transportation. There are not enough jobs either, so many people live in poverty.
Peak oil is the point in time when the maximum rate of petroleum extraction is reached, after that the rate of production is expected to enter a decline. Eventually we will have basically no oil, so we need to figure out how to transport food, resources, even people without using fossil fuels.
We don't need to kill people to fix overpopulation. If somehow the government limited the children you can have to 1 (only replacing yourself) then the population would stay the same for a while. Then slowly decrease, because not everyone will have a kid, only some people.