And so to me, the key is not to actually give him a character. He's devoid of it because he doesn't care. Darth Vader does not care. He does not have compassion. He does not see you. He sees the thing he wants to destroy, and he will do that. It's like the same feeling you got in Rogue One when he comes down the hallway. He doesn't talk to those guys. He's going to destroy them. He has one mission, and all of his remorse and all of his anger and all of his hate is in every swing that he does. That's how it’s resolved.
https://www.yahoo.com/entertainment/tv/articles/dave-filoni-perfectly-explains-why-103131368.html
Another thing to consider is that we don't really know young Darth Vader. Apart from his final moments in Revenge of the Sith, this show taking place about a year after, Vader might not be much of a talker yet. I think comics explore this a little more, but Vader may not be anything more than a ball of anger at the moment.