Thanos, as portrayed in Avengers: Infinity War, proved to be one of the most compelling villains the Marvel Cinematic Universe has ever featured. A purposefully cold figure who had knowingly hardened himself to achieve a task he believes is worth the sacrifice, his motivations still don't offset the terrible acts he committed in his pursuit of universal equality. It's a tricky balancing act to make an intergalactic mass murderer sympathetic, but it's something the people behind the film managed to pull off with gusto.
Throughout the film, one of the best ways Thanos reveals his sense of self and thereby differentiates himself from other more bland universal conquerors is in the genuine respect he develops for the various people he fights. That aspect of his character forms the backbone of a new fan theory, explaining why Thanos was willing to fight Captain America hand-to-hand instead of just obliterating him on the spot -- and it makes perfect sense.
When A Plot Hole Is Really Just A Character Beat
By the climax of Infinity War, Thanos has arrived on Earth with five of the six Infinity Stones. All that stands between him and the final one is the remaining beaten and battered champions of Earth. With five stones, Thanos was ripping chunks out of moons and altering reality around him to counter Doctor Strange. The odds of him actually being threatened by the handful of humans (and teenage Groot) standing between him and the stone in Vision's head are minuscule. So, how do any of them survive his wrath?
A new theory on Reddit explains that Thanos doesn't just tear through the few remaining heroes because he simply doesn't want to. The idea focuses on how Thanos merely incapacitates the heroes throughout the film instead of killing them straight out. The only ones he genuinely targets are the ones that get in his way. When he kills Loki and Heimdell, it's less of a vindictive act and more just to respectively remove a traitor and a teleporter from the forces opposing him. The rest of the heroes who threaten him? They get beaten down, but never to the point of death. There's a decent amount of evidence to support this theory throughout the film.
Hulk tries to assault Thanos and the Titan knocks him out, but never moves to fatally dispatch him. When he challenges Star-Lord to kill Gamora, and he actually tries to go through with it, Thanos tells Quill that he likes him for being willing to follow through. Even when he's going all out against Iron Man and willing to wound him, Thanos admits an honest respect for the man beneath the suit. He tries to incapacitate his enemies, and is only willing to kill them if he needs to, much like the heroes do.
He doesn't rip apart the Earth as he makes his way towards his target, even though he easily could. He doesn't blast them away with the kind of power he was shooting at Doctor Strange or Iron Man. He lets them throw whatever they have left at him, and then just knocks them to the ground and continues on his way. He binds War Machine, traps Black Widow and knocks out Steve Rogers. He's not actively trying to kill any of them because he sees that as unnecessary.
It reinforces the sentiment that he genuinely loves Gamora, and that makes his villainous turn more tragic. The theory suggests that very respect carries over to Steve Rogers, the man rushing up to Thanos and willing to go toe-to-toe with the Infinity Gauntlet with nothing but his fists, grit and determination. It's a perfect explanation for why Thanos doesn't go over the top against the Avengers, and reinforces the more relatable aspects of his character. This is the perfect explanation for why Thanos doesn't kill Steve Rogers, turning a potential plot hole into a great character beat.