How we cite our quotes: All quotations are from Star Wars: A New Hope.
Quote #1
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.
Quote #2
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.
Quote #3
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.