Even though Anakin becomes evil at the end of the movie, his personality, his pattern of speech, and his mannerisms in his "evil" state don't seem to line up with how he is in the suit as Darth Vader. There should have been a better way of showing the physical and emotional transformation of Anakin into Darth Vader as a character.
lucas spend 2 week write his script and 2 years on special effect (yes look it up, the prequel film TWO year before they released in cinema so lucas can fill holes in script with CGI).
I can believe it, because both Anakin and Vader are idiots!
Of course Anakin is an idiot, one who fucks everything up and who obviously lets his feelings override whatever intellect he may possess, but Vader is also kind of dumb. He doesn't check on Palpatine's story of Padme's death, he lets Palpatine dominate him for 20 years without thinking about whether that's a good idea, he solves every problem he's confronted with by threatening or hurting people rather than thinking about the problem, he never notices Obi-Wan hiding in a totally obvious place, etc. He's still someone who's letting his feelings override whatever intellect he may possess, and anger and self-pity are still the most dominant feelings...
I always found it funny that he never knew about Luke or Obi-Wan. He has the same name and Obi-Wan goes and hides on his home planet lol. All the extra movies and series has now made it seem even more stupid. Turns out the Empire was on Tatooine all the time looking for rebels or force users, but never bothered to check out the Skywalker Ranch. What?!
That's the problem with writing the backstory retroactively, and doing a half-assed job. A devoted fandom is far more thorough and invested than any Hollywood writer!
So yeah, the Jedi are declared Public Enemy #1, and now there's this whole organization set up to hunt anyone with Force powers down, and nobody ever checks on people named Skywalker or Kenobi to see if they might be of interest? Nobody ever checks on any Skywalkers from the Mos Eisley area, to see if anyone else in the gene pool might have a bit of Force ability? Bitch, please!
A lot of OT fans (and some prequel fans for that matter) really didn't like the man-child whom they called "Whineakin Skywalker," and were unhappy for the same reasons you are; that the "human" Anakin doesn't seem to have anything in common with the cyborg Darth Vader. My own gripe is the fact that Hayden Christensen can't act his way out of a paper bag. He was the weakest actor in the entire cast in Episodes 2 & 3, and was only chosen because he looked the part and could do stunts; that's it. Lucas is apparently not as good a writer as he pretended to be back in the 70s and 80s, back when he took credit for better writers' work on the scripts.
However, when comparing him to Crylo Ren, Anakin still wins the race of who's more popular, despite the bad acting.
My age has nothing to do with it. Hayden Christensen sucks ass as an actor. He's as whiny, wooden, and unemotional or as poor at emoting as Robert Pattison and Kristen Stewart combined. He was no different in "Jumper," and that was a later film he made, in case you didn't know.
It makes me wonder how low your standards are for what a "good performance" is.