Might be a little bit of a hot take, but the youngling scene is only bad because of how abrupt it is. It’s not a new low for Vader when you really put it in perspective.
Darth Vader, even in the OT in isolation, has the blood of thousands on his hands. Rebels, Jedi, innocents. He embraces the dark side and was even straight up sadistic. He knows he’s a monster and embraces it. It’s not like when Tarkin was gonna blow up Alderaan, he was like, “But dem kids”. His heart was frigid at that point.
Anakin’s redemption was never about making up for his crimes. He was a cold Sith for decades. It’s spiritual, stemming from the ounce of good left in him, his love for his family. It’s never too late to make the right choice. The fact that the line was crossed a long time ago is the point.
To me, that’s an important part of Vader’s character. He’s a villain. I love him in a “he’s terrifying and badass” way, whilst also having a deep connection to his depth as a character, not “what if he’s actually right, society amirite”. I don’t sugar coat his actions.
The Prequels just didn’t do a good enough job getting us to the point where him killing children comes across as anything other then insanely abrupt.