There is a central moral question at work in Avengers: Infinity War between the protagonist, the villain Thanos, and the multitude of antagonists, the Avengers, Guardians, and various other Marvel heroes. The goal of Thanos (no spoiler here as the trailers explained it months ago) is to wipe out half of the life in the universe in order to "balance" life and usher in an age of abundance and peace. At the root of the motivation of Thanos is the fear of overpopulation (and with it environmental degradation) leading to suffering and strife over limited resources. In other words, in order to significantly decrease suffering and increase happiness, countless sentient lives have to be snuffed out. Thanos believes that the ends justify the means (his goal is worth killing over), conversely the various heroes reject this moral equivalence, maintaining the sanctity of all life.
Fear of overpopulation is nothing new for humanity, in 1798 the British economist Thomas Robert Malthus published An Essay on the Principle of Population which predicted that population would double every 25 years but agricultural increases could only be incremental, thus resulting in widespread famine and war unless significant birth control measures were to be taken. Malthus' warnings influenced many, among them the German imperialists who contended that Germany needed Lebensraum ("living space") to accommodate its growing population, inevitably at the expense of Germany's Slavic neighbors to the east who would need to be eliminated or turned into serfs. The unforeseen agricultural revolution that followed after Malthus' dire predictions made the billions of human beings living in the 20th century possible, although fear of overpopulation remained, typified by the sci-fi movie in 1973, Soylent Green. With the population of the world in 2018 at 7.6 billion and rising, that fear isn't likely to go away anytime soon, thus the question remains: How much of a problem is rising population, and what is the moral response to it?
As stated earlier, the response of Thanos to the fear of overpopulation is genocide, a willingness to kill in war, planet by planet, half of the population, and the hope that he can obtain all six of the infinity stones for his gauntlet and then finish his task across the universe with a "snap of his fingers". The heroes in Infinity War are faced with the question of the value of life on a much smaller scale as they must contemplate self-sacrifice in order to attempt to stop Thanos. While Thanos was willing to kill on an epic scale to achieve his goal, the heroes must be willing to risk their own lives, a question whose consequences they face multiple times in the movie.
The self-sacrifice of an individual to save many is certainly a theme embraced by Christianity, it is after all what Jesus Christ did when he accepted the task of dying upon the cross as a sacrifice for the sins of humanity. In order to save billions, Jesus willingly carried the cross upon which he died. Thankfully, that sacrifice was not in vain, for with his resurrection he obtained victory over both sin and death for all those who would believe in him.
It will not be known until after Avengers 4 (set to release in 2019) what the final cost of confronting and possibly defeating Thanos will be for the Marvel heroes, but the principle established by their decision to oppose him is one in which life is considered a precious thing, and while self-sacrifice may prove necessary to stop great evil, it is not a decision to be made casually precisely because life is precious.
