Warning: this is a very spoiler-heavy analysis. Do not read this unless you’ve seen the movie. (I’ve already seen it twice!)
Consider these key details from the film:
- Vision tells Wanda to run when they’re attacked and he’s badly hurt, but she refuses, joking that in a romantic moment before the fight started, he had asked her to stay.
- Vision later says that the stone in his head must be destroyed, even if it kills him, so that he can ensure the safety of others. Steve Rogers declines that plan, saying that “We don’t trade lives.” Later, when Steve tells Vision to run so he can save him from death, Vision stays and kills the attacker, saving Steve instead. Vision then tells Steve, “We don’t trade lives.”
- In a twist on this theme, Gamora asks Quill to kill her if she’s taken by Thanos, so he won’t be able to use her to hurt anyone else.
- In fact, both Quill and Wanda are forced by people they love to kill them in order to save others. In each case, they hesitate and only do so with extreme pain to themselves evident on their faces.
All of these are examples of self-sacrifice, motivated by love and honor, but there’s a third instance of someone killing a loved one in the film.
Thanos, of course, kills Gamora, but not to save others, and not at her request. Where all the acts listed above were voluntary and selfless, Thanos acts against the will of others, for his own selfish wants.
But this isn’t a generic theme of loving sacrifice here. Consider three other moments:
- Loki, in a singularly unusual act, attempts to kill Thanos after noting his relationship to Odin in an aside to Thor, to stop Thanos from torturing Thor to get what he wants. Loki dies as a result. The only motivation the generally selfish Loki could have had for this is to save others, probably a result of character growth in Thor: Ragnarok.
- In a parallel scene of saving a sibling from torture, Gamora gives up secret information to Thanos to stop him from torturing her sister Nebula, with whom she had reconciled in Guardians of the Galaxy Vol. 2.
- We first see Tony Stark in this movie as he enthusiastically tells Pepper about a dream where he would become a father. Though there’s no baby, Pepper says, Tony spends the rest of the movie becoming an ever closer mentor to Peter Parker, even holding the younger man as he dies in Tony’s arms, which devastates him. That’s why there’s no Uncle Ben in the MCU: Tony is Peter’s surrogate dad here. [This is another missed opportunity by DC, who created their current franchise with an aging, grizzled Batman and a young, inexperienced Superman. A lot could have been gained by having Bruce Wayne become a father figure to Clark Kent. Alas.]
- Thor and Quill share some darkly humorous banter about the stresses of having family members kill other family members, obligating you to kill them in return. In fact, with Gamora’s situation as the focal point there, they bond over it.
All of these details emphasize family, especially the power of love in a family. Thanos is the villain here because he is the one character who perverts that power and twists it backward for his own good. Everyone else sacrifices for family love. Thanos abuses it for his own glory. In the moral universe of Infinity War, that’s what makes him evil. And sacrificing yourself out of love for family is what makes the heroes truly heroic.
Not long ago, I saw 
Gently Hew Stone 