It's not that Capaldi is some stratosphere-level über-Olivier, although he's an excellent actor. It's just that Warren is so awful. Every line is delivered in the same deadpan monotone, probably meant to create at atmosphere of menace by calling to mind some serpent's sibilant hiss but managing only to drone his victims into a stupor. Every gesture is mannered and artificial, like a Mark II Villain Droid (Swordfighting Model). I have no idea what other work Warren has done or how well he did in those roles, but I have to believe that, given the long list of his credits, he's done better elsewhere.
The series is not helped by a lousy mid-season. "Through a Glass Darkly" gives us the old standby of a villain who insists his victims "play a game" that is meant to satisfy his own twisted sense of justice. But it can't carry its conceit through the whole episode and relies mostly on demanding King Louis (Ryan Gage) cry like a binky-less toddler. Seen in series one as a lightweight and rather unserious dilettante under the Cardinal's thumb, here we have a Louis who is petulant, paranoid and more or less incapable of finding his own thumbs. "The Return" suggests Athos (Tom Burke) cares so little for the tenants left behind on his family's land he has to be shamed by his comrades into helping them, even though nothing about what we've seen of him until now indicates he's that big of a jerk.
Other characters fare about as well. D'Artagnan (Luke Pasqualino) spends the first half of the series pouting because his true love Constance Bonacieux (Tamla Kari) won't leave her husband and be with him openly, even though said husband won't divorce her and D'Artagnan has a rather dangerous profession and could leave her without lover or family security. Constance isn't the plucky heroine who wanted to learn how to shoot and use a sword; she's a simpering damsel powerless before Rochefort's machinations. Queen Anne (Alexandra Dowling) is no longer a player in the palace power struggle -- she's also a simpering damsel powerless before Rochefort's machinations. Aramis (Santiago Cabrera) is now a swinish lout who, having previously bedded the queen and holding her as his One True Love, endangers her and the son he secretly fathered on her by insisting on hovering near them and callously dallying with the prince's governess Lady Marguerite (Charlotte Salt) solely in order to do that. His casual affair with and rejection of Marguerite gives Rochefort a lever over her and eventually results in her suicide, an event which draws not so much as a shrug from Aramis or his comrades.
Plot holes abound. Milady de Winter (Maimie McCoy) schemes for her own ends, but when given the gift of an assassin and spy she might be able to use to play against Rochefort and regain influence with the king she instead inexplicably kills the woman. The testimony of the king's loyal officials can't convince him Rochefort, his top minister, is against him, but the word of the Spanish spymaster will (this exchange wisely happens offscreen, as series two finale writer Simon Allen probably thought, "There is no way I can sell this conversation without invoking alien mind control.")
The Musketeers isn't meant to be taken seriously. It's one big suspension of disbelief from its impossibly-accurate-for-the-time weaponry to the 21st-century morés displayed by its characters, but suspending disbelief is the bargain you make with the show when you watch it. The viewer agrees not to remember that smooth-bore 17th-century muskets were accurate to maybe a hundred yards and handgun-sized models even less, and similar things. In return the show agrees to offer intrepid heroes who swash some buckles, buckle some swashes and do some derring in a carelessly entertaining manner while fighting mustache-twirling Evil Villains with their Villainous Plots. The carelessness is not supposed to show up as inattention to detail that renders our nobles ignoble, our heroes jerks and our villains crashing bores.
BBC has renewed The Musketeers for a third series. Rochefort's death means a blessed absence of Warren, so who the Musketeers' main opponent will be is up in the air. There are a couple of intriguing possible storylines, as Milady struggles with having become a person she despises and Athos becomes aware hate is not all his heart holds for her. The bluff and no-nonsense Musketeer Captain Treville is now a government minister, smack in the middle of palace intrigue. Playing those out, though, will need some more attention to what made the characters popular with viewers, what makes heroes heroic and a recognition that "snoozing with fear" is not the response you want your lead baddie to evoke.