Thanos—the Eternal Titan, ruler of the universe—wore many crowns: collector of the Infinity Stones, the man seated upon the throne.
He was also known as the director of the Cosmic Family Planning Committee, whose full-time duty was maintaining the universe's harmony and balance.
He believed that the universe was finite, and its resources were finite as well. If life was left unchecked, extinction would ultimately be unavoidable.
William actually agreed with him on that point. Back then, he had discussed the Dark Forest Theory with the Avengers.
That had been on the eve of the Battle of New York, when S.H.I.E.L.D.'s Phase Two had sparked a dispute among the Avengers.
If the entire universe was a vast dark forest, then every civilization was a hunter carrying a gun.
The Dark Forest Theory had two important axioms: first, survival was civilization's primary need. Second, civilizations continuously grew, developed, and expanded, while the total amount of matter in the universe remained unchanged.
This was the same as Thanos's view. In order to survive, civilizations would inevitably reproduce, develop, and expand without end—and war was born from that.
Whether within a civilization itself or between civilizations, where there were people, there would be conflict, suspicion, and ultimately war.
Every great villain had a grand goal, and Thanos was no exception. His goal was to maintain the balance of the universe by wiping out half of all life.
He had originally slaughtered his way across planet after planet, carrying out massacre after massacre, his task being to kill half the population of each world. But that was far too slow, which was why he wanted to collect the Infinity Stones. As long as he gathered every Stone, he could snap his fingers with the Infinity Gauntlet and achieve his goal.
Half the life in