Why doesn't it surprise me that you smoke weed?"What's the motivation for these people?"
There is one secret society that historians agree on had a "grand plan" to take over the world - the Illuminati, founded in 1776 and banned in 1784 (though Weishaupt said that he had a plan even if they were discovered; you be the judge if that happened).
According to Weishaupt, the founder, their motivation was that they "deserved" to rule the world: according to themselves, they were the descendants of the "fallen angels" that lived on the forgotten/destroyed continent of Atlantis and later interbred with humans, creating a race - supposedly superior - called the "Aryans", who where taught hidden, ancient wisdom by these "fallen angels".
As a "superior race", they believed that they should rule mankind and that everyone should worship the same entity (Lucifer) that both they and previous secret societys worshipped, getting rid of these "tyrannic" and "despotic" religions that were currently in place.
Some conspiracy theorists do believe that these "fallen angels" were "aliens". Personally, I am in the other common school of conspiracy belief - that these people ruling us today are the descendents of a tribe banned from Babylon in ~600 BC because of their hedonistic beliefs. Its important to understand, because many read this the worst possible ways, that I dont believe Jews to rule the world - rather the reverse, a people banned by the Jews.
While these two common perspectives on "origin" vary, we believe their motivation is to control us "lesser humans" and have us worship Lucifer. There could be additional motives, tied to gold or energy, but ths another lengthy story.
"What would be the point of all this population control, killing people stuff?"
Some believe that they take certain pleasures in killing, that they can harvest something they find valuable (adrenochrome) out of dead bodies or to make ritual sacrifices to "fallen angels" like Moloko.
Most of us however believe that they think the world is overpopulated. The number of people is a threat to the planet they want to rule. These people are probably no more than a few thousand - and they dont need 7 billion slaves. A more likely number, perhaps getting a lot of attention because of the Georgia Guidestones, would be 500 million.
So in short: most likely to balance human population and nature in a sustainable way. As for control, it is necessary to implement that before any mass killing, since that could be chaotic in a potentially uncontrollable way. Chaos is only good when it leads to order.
"Who is actually gaining if that is real?"
Their own "race", carefully intermarried through the centuries. Your ordinary low degree Freemason or Rosicrucian or Thelema member might think they are to gain something, but most of us believe they will be toast or enslaved when the new order is finally implemented