Reefer Madness with superheroes
The new Avengers movie is well made. The special effects, acting, action and editing were all well done. Despite the fact that the movie is two and a half hours long, it does not feel that long because of the multiple plot threads going on at once. That said, I cannot give it a top grade because of the eco-terrorist nonsense that flows throughout the film - and a massive plot hole that invalidates the movie.
In the comics, Thanos was literally in love with Death. (In the Marvel Universe, Death is a sentient being who looks like a busty brunette Caucasian woman.) Thanos gathers the Infinity Stones into the Infinity Gauntlet and murders half of the universe to impress her, although everyone comes back to life at the end of the story. In the movie, Thanos wants to murder half of all sentient life in the universe because of overpopulation. He is basically the Batman villain (and fellow eco-terrorist) Ra's Al Ghul on a cosmic scale.
Here is the problem. Without getting into whether the Population Bomb actually exists in the real world or not, it is at least plausible in the DC universe that overpopulation threatens planet Earth. This makes Ra's Al Ghul's motivation believable, even though his methods are evil.
On a universal scale, though, that is utter nonsense. You cannot tell me that in the entire universe (including uninhabited planets) there are not enough natural resources for life to exist at its current level and even much bigger. I can suspend disbelief to enjoy a movie with gamma monsters, super-powered armor and Asgardian gods, but the idea of a universe-wide lack of natural resources is stupid.
Here is the plot hole. Thanos eventually gathers all of the Infinity Stones into the Infinity Gauntlet. He has unlimited power and he can rewrite reality on a whim. So why doesn't he just use the Infinity Gauntlet to make sure that there are enough resources for everyone forever? If "saving the universe" from overpopulation was Thanos' goal, the entire movie should be over in five minutes, because the Avengers should be helping him. He assembles the Infinity Gauntlet with the help of the heroes, solves the Universal Population Bomb, and everybody lives happily ever after.
But it actually gets worse. The way the movie presents things, Thanos is right. He murdered half of the population of Gamora's home planet. Now that there are fewer people, the planet is prosperous and everyone is happy. Thanos brags that every child now goes to sleep with a full belly. Of course, this is utter nonsense, because if half of the population of a planet were annihilated it would throw the entire global economy into complete chaos. The planet certainly would not recover in the time it takes for a small child to grow into an adult. But if we can suspend disbelief to accept this, the movie does nothing to show Thanos was wrong.
Also, it is important to note that Thanos' goal was to murder half of all sentient life in the universe, not animals. This is an extension of "humans bad, nature good" propaganda from militant environmentalists. One wonders if the people writing the script actually agree with Thanos, and not the Avengers. Had the motive for Thanos been anything other than overpopulation this would get a higher grade.
Final Grade: B-