Both Kylo and Anakin are driven by a lot of the same feelings: anger, entitlement, selfishness and obsession. They lash out too easily when provoked; they yearn to control the uncontrollable and they allow grudges and betrayals to fester and poison their souls. But while Anakin's turmoil feels overwrought and one note, Kylo's feels nuanced and unpredictable. Anakin's rage is filtered through petulance and whining complaints, whereas Kylo's modulates between immature outbursts and quiet moments of lonesome contemplation. Anakin's motivations are painted in broad, two-dimensional strokes, whereas Kylo's are multi-layered and fluid, like a real person's.
This aura of mystery is a huge part of what made Vader such a strong villain in the original series; something the Prequels stripped far too much of away. It's a hard balance to strike between offering satisfying answers to burning questions and holding enough back to tantalize an audience's curiosity. In a way, it's unfair to lambast a series for not being unpredictable enough when we already knew the ending while praising that same trait in one that is able to progress the Star Wars canon into unknown territory. But, as Rogue One proved, just because we've already seen the destination, it doesn't mean the journey can't still surprise us.
The Sequels also arguably sink Kylo into deeper darkness by making his crimes have a greater emotional impact on us. Though the youngling murders are monstrous, Anakin kills off characters who we have little attachment to, like Mace Windu. (Sorry, Sam Jackson.) Kylo repeats the youngling murder spree, kills his own father in cold blood and comes to close to blowing his mother up as well -- two characters who fans have very strong attachments to. One of Anakin's tipping points was his mother's death, but that was something that happened to him. Perhaps if the Prequels had had the nerve to make Anakin himself deal the finishing blows, we'd have been much more convinced by the depth of his darkness.
The biggest mistake that the Prequels make, however, is that they suffocate Anakin's character arc under the convoluted weight of the rest of the story when it should have been the driving force. The Last Jedi is particularly successful at allowing character to steer plot rather than the other way around, the latter of which makes characters act in ways that feel unnatural and forced. With enough focus, breathing room and nuance, Anakin Skywalker could -- and should -- have been the convincingly dark, complex and intriguing villain that Kylo Ren is.