Sadly, bad writing is a problem in the game, not Simmons. If it were the actor's fault, it wouldn't affect the game that much.
Kethric just seems really uninteresting villain. It seems we're treated to yet another clueless pawn who is thinking he can somehow realize his dreams by serving the absolute. Or, given his subdued tone, maybe Kethric just goes through the motions, and is a self-aware pawn. At best he might have some information on who is behind the flayer cult, but that whole "Death's design" portion of the storyline seems like a convoluted mess, that's seemingly there to make using the tadpole less fatal. There just doesn't seem to be much, if any, person underneath Kethrics misguided singular fixation, which makes his family life monologue a bore.
Also, it's not like there's a short supply of (male) characters in fiction/reality that go on a murderous rampage after losing their loved ones, or their ability to love anything. Sure, you can make such a character interesting, but the lines about his family are about as awkward as a badly written voice over. I'd not expect someone like him to just open up to a perfect stranger/enemy he's presumably just met about his inner life in such stunted detail.
It's also kind of strange how non-frightening they made him into. I'd say even the redcaps or the gnolls are more scary than him. You'd think that with the game setting being firmly so in body horror, they'd present a necromancer general as foreboding figure; maybe zealously preoccupied with his coping work instead of just idly moping on his throne in a party get up. The only horror element prominently on display in the trailer is Larian's "bow to the tadpole" -fixation, which is just cringe.
