A man dressed in all black enters the battle's aftermath. His faced is covered with a mask, and his mechanical breathing can be heard in the silence. He looks briefly at the dead bodies littering his path before stepping over them and entering the ship.
It's not good practice to judge appearances, but this dude is totally a bad guy. Before we realize this is the Darth Vader mentioned in the text crawl, we can tell he's an old-school villain that does dirty deeds dirt cheap.
OBI-WAN: A young Jedi named Darth Vader, who was a pupil of mine until he turned to evil, helped the Empire hunt down and destroy the Jedi knights. He betrayed and murdered your father. Now the Jedi are all but extinct. Vader was seduced by the dark side of the Force.
Notice that Vader's seduction to the dark side isn't described in terms of a character flaw or a result of the war changing him. He simply "turned to the dark side," as though going from good to evil is like flipping a switch in your soul. This phrasing really shows the binary morality on display in Star Wars.
OBI-WAN: I need your help, Luke. She needs your help. I'm getting too old for this sort of thing. LUKE: I can't get involved. I've got work to do. It's not that I like the Empire. I hate it, but there's nothing I can do about it right now. It's all such a long way from here. OBI-WAN: That's your uncle talking. […] LUKE: Look, I can take you as far as Anchorhead. You can get a transport there to Mos Eisley or wherever you're going. OBI-WAN: You must do what you feel is right, of course.
Grandpa Obi-Wan's tone suggests, of course, that it isn't right. In Star Wars, the only response good should take toward evil is to ensure it is defeated because the Empire is super-mega-ultra evil. Luke's dropping the ball here.
TARKIN: You would prefer another target? A military target? Then name the system. I grow tired of asking this, so it will be the last time. Where is the rebel base? LEIA: Dantooine. They're on Dantooine. TARKIN: There. You see, Lord Vader? She can be reasonable. Continue with the operation. You may fire when ready. LEIA: What? TARKIN: You're far too trusting. Dantooine is too remote to make an effective demonstration, but don't worry. We will deal with your rebel friends soon enough. LEIA: No!
If you thought Vader was evil, get a load of Tarkin. He blew up an entire planet simply to prove that he could blow up an entire planet. He's what would happen if a Bond villain were elected president. Dude is just evil.
HAN: It is for me, sister. Look, I ain't in this for your revolution and I'm not in it for you, Princess. I expect to be well paid. I'm in it for the money. LEIA: You needn't worry about your reward. If money is all that you love, then that's what you'll receive. LEIA [to Luke]: Your friend is quite a mercenary. I wonder if he really cares about anything or anybody.
If any character comes close to toeing the good-evil line, it's Han. His motives are self-centered, but his actions are praiseworthy because he fights with the heroes. For an adventure story like Star Wars, actions are what count.
HAN: You're all clear, kid! Now let's blow this thing and go home!
Was there ever any doubt? Despite being a bit on the fence earlier, Han lands on the side of good. In fact, all the characters in the original trilogy ultimately choose good or evil in the end, no matter how wishy-washy they were before.
Luke and Han are presented medals for their heroism during the Death Star assault. The Rebel forces applaud as the triumphant music swells in the background leading to the credits.
It's a classic ending for a traditional good versus evil story. The good guys stand triumphantly; the bad guys have been defeated. It's good feelings all around. Unless you're Wedge. He got shortchanged in the hero ceremony.