Let’s see… where to begin…
Between 1977 and 2016, Star Wars went from a stand alone film to the 4th film of the saga. So character motivations changed a lot. We got Anikin and Obi-wan’s back stories. But even back in 1977, Vader wasn’t the real bad guy of the film, Tarkin was. Vader was the face of evil. The man on Tarkin’s right hand. An independent opearator working for the Emperor and assigned to help Tarkin. A man on a mission to find the missing plans (which he failed to do, though his plan to track Leia to the rebel base was genius).
So let’s look at what Vader did in the OT. He did not lead the attack on the Tantive IV. He sent the stormtroopers to take the ship. With the ship captured there was no place to run. The gunners were on task to destroy any escape pod so no one could get away. They didn’t destroy the one because it had not life forms an they didn’t have orders for that.
In TESB Vader is leading the attack on the Hoth Base. We don’t see him even draw his light saber… because the base has been abandoned. There is no one there. He reaches the Falcon just as it takes off.
In ROTJ, Vader is under the personal command of the Emperor. He is reduced from being the big bad guy to being a servant. He is following orders and frankly is not nearly as scary as Palpatine. He fights Luke at his master’s orders.
Now, what else do we have to show how Vader might fight with a light saber. Well, we the PT. In three films we see how Jedi fight battle droids. They chop them, they dice them, they pick them up and throw them. And after his turn to the dark side, the first thing Vader does is slaughter the younglings (and who knows how many padawn, knights, and masters) in the temple.
So we come into Rogue One with a working knowledge of how Anakin/Vader fights and what tactics he would use. What we see in the Hallway is EXACTLY that. He has no chance to do that in the OT. But in Rogue One, he is on the trail (for his masters the Emperor and Tarkin) of stolen plans and their recovery is vital. So he will stop at nothing to get them and it is too important to leave to stormtroopers. So the hallway slaughter makes total sense from what we see. This is the powerful and unstopable Lord Vader whose reputation preceeded him in the OT. It is events like this that made him feared. We see very little in the OT that inspired fear. Vader has been a popular bad guy since he first appeared in 1977. The hallway scene is the first chance to see Vader in action. It bridges how Anakin slaughtered battle droids to the reputation Vader has in the OT. And it also fits with how Obi-wan likes to disarm people.
People have been cheering Vader since 1977. The reaction to his appearance in Rogue One is just a natural continuation of that.
The end of Rogue One doesn’t really alter anything about A New Hope. The Tantive IV was not seen in the battle. The ship drops out of the hanger and flees and Vader has to get back to his ship and track them down. There may be no proof of what ship it is (they are a common class of ship after all). And the nature of the plans and how they got into Leia’s hands doesn’t contradict A New Hope either. Leia physically inserts the plans into R2. Sure there was a transmission, but it was put on physical media. So the spies transmitted the plans and Leia had them and is hiding where they went. Does it exactly match the dialog? Probably not, but neither do other conversations in Star Wars and so many in real life.
As for the word spies. Rogue One is, underneath everything, a WWII under cover mission movie where everyone dies. The Vader and Leia scenes at the end exist to give fans an uplifting ending after watching all the Rogue One team members die various deaths. In a time of war, any operative working for the other side is a spy. I never pictured a James Bond type of spy situation. I always pictured a team like we see in Rogue One, though I never though about if they had lived or not.
So I think Rogue One fits perfectly between ROTS and ANH. It adds to ANH without detracting or contradicting anything. It showcases Darth Vader as a badass who has built a reputation as a feared figure. It shows us a team of rebels who steals the plans and the very successful battle that most of the rebels went home from. They lost a few fighters and one capital ship (which was staying to receive the transmission), but most of the ships went home and the ones that went back to Yavin IV would soon fly their next mission against the Death Star.
And I feel it is very appropriate to cheer a bad guy when he is doing something cool and you know how his story is going to end. It doesn’t mean you are sick in the head, only that what you are seeing on screen is really cool and really fleshes out the character even more. And Like I said, what Vader did in that hallway is what we saw Anakin do many times (many many many times if you watch Clone Wars) before to battle droids. Why would the evil dark Lord of the Sith treat rebel soldiers any different than battle droids? That hallway scene to me, cemented Vader’s reputation and how committed he was to the cause and how evil he had become. And so much of that was only conveyed by Kenobi in dialog and never shown before, “a student of mine until he turned to evil” and “he betrayed and murdered your father.” So it is a glorious spectacle that so many Star Wars fans reveled in, not because they don’t side with the rebellion, but because we got to see that evil brought to life and see how truly feared Vader should be.
And as for Rebels, they do not have success after success. They have many successes, but they are all small. No serious conflicts. Lothal is only freed after many setbacks and the cost of one of the main characters. And again, we get to see Darth Vader do what Kenobi said he had done, hunting down and trying to destroy Jedi. So everything in Rebels and Rogue one goes back to what we found out in 1977 and was elaborated by the following 5 movies.