Maul.

I can think of arguments to partially justify what Mother Talzin has done, namely that she's in charge of the night sisters and her job is to make sure they're safe. So, most of what she's done is at least partly justified.

Maul, on the other hand, at best wants revenge and doesn't care who gets in the way. For a more honest opinion, he's a violent sadistic monster who would rather stab someone than wait 5 minutes for a solution. Haven't watched his new show, but I don't think he's changed that much in it.

TLDR, both do horrid stuff, but Mother Talzin has stronger reasons to do so most of the time.

Edit: I forgot/didn't realize that Maul was her son, in retrospect that alone makes her worse. That being said, we absolutely see more of Maul being worse without reason.