There are many terrifying things about Darth Vader. I could go into detail about his intimidating appearance and voice, or about how he slaughtered the halflings without a second though. I could also cite how he tore through the Rebels aboard Leia’s ship, or killed numerous Sandpeople. All of these things are factors in how evil the Sith Lord is, but what is the most terrifying thing about Darth Vader?
The most terrifying thing about Darth Vader is not his power, or his ability to slaughter and maim at will, it is what could-have been.
Vader could have been the most powerful Jedi to have ever existed, but the Jedi Counsel was afraid to let him live up to his potential. This was their big mistake. They kept him at arm’s length. They didn’t want Anakin to be trained at all, because of the belief that it was too late to train the youngling.
Yoda showed this hesitation when Luke Skywalker approached him, but he saw something different in the young Force user. He saw Padme. He saw the love and good she possessed in Luke. This is the only reason he decided to follow through with Luke’s training.
It was this distance that allowed hate and anger to consume the young Padawan right under their noses. Had the Counsel not been afraid of what he could become, they would have sensed the turmoil inside of the young Jedi. They would have saved him. It was this distance that also allowed Palpatine the opportunity to corrupt Anakin.
When he turned, we got to see that unrestrained anger in his face-off against Obi-Wan Kenobi. His anger blinded him (literally) even when Kenobi pointed out that any further attack would be futile for Anakin.
When Palpatine saved Anakin and put him in the iconic Darth Vader suit, it wasn’t a suit designed to just keep Skywalker alive. It was also designed to keep his force powers restricted. It kept him from ever reaching his full potential, so that he could never best his Master.
His potential is the most-terrifying thing about the villain, mainly because no one knows what he could truly become had he been nourished, and trained to the fullest ability of his teachers. The Jedi kept their distance, leaving his training in the hands of a young Kenobi, offering little-to-no assistance.
Emperor Palpatine did the same thing, keeping Vader restricted in the suit. His hate and anger partnered with his potential scared the Sith Master. Fear kept Anakin restricted in his training on both sides of the Force.
How formidable would Vader have been had he been able to use his abilities to their fullest potential? He would have become more of a legend than he already was. He was the Chosen One, and he eventually brought balance to the Force, leaving just one Jedi and One Sith remaining. (I still consider Vader to be a Sith when he died, although he was redeemed by Luke at the end of Return of the Jedi.)
When Vader died, the Force was once again unbalanced. Luke’s Jedi Academy was destined to fall due to this unbalance. Vader was a powerful villain, but unrestrained, he would have been so much more.