“Mary Kills People,” one of the cult gems of television, has a more somber mood in its second season. A bit of the fizz and crackle of Season 1 has been replaced — understandably — by a tone that adeptly mixes a slightly harried mood with a somewhat melancholy atmosphere. As the consequences of Mary Harris’ shady side hustle pile up, it’s only natural that she and her partner in crime would start to reconsider everything they’ve done in the name of compassion. Could it be that there something else mixed into their signature concoction, and it wasn’t just a pure desire to help that was driving them the whole time?

The part-time job shared by Mary and her best friend Des (Richard Short) is helping the terminally ill end their lives. Usually these events takes place at the home of the person who has elected to end his or her life, and the mood at these occasions is quite often one of of bittersweet but determined resignation. Mary is an ER doctor, and she has a prosperous life that involves an attractive seaside home and two lovely daughters. She and her husband have split up, but if she wanted to, she could disappear into suburban safety and maybe even settle for a slice of predictable contentment.

But something else drives Mary; she’s not one for bonfires on the beach and PTA committees. One of the most successful elements of this canny series lies in the fact that the writing and Caroline Dhavernas’ performance never explicitly spell out why there is darkness in this complicated woman, and how much more of it lurks in the corners of her psyche. Last season, but even more so this year, we get hints about the difficult past that made Mary a fan of control and power. Growing up, one gathers, she didn’t have much of either, and as an adult, she’s making up for lost time.

The problem is, some of the decisions Des and especially Mary have made along the way have begun to make their lives spin out in unexpected ways. Des is a former drug user, and even though he’s more or less clean now, both of them share an addiction not just to their unsanctioned hobby but to the adventures and dangers that often accompany their errands of mercy.

To ease the suffering of the terminal patients she helps, Mary has to get drugs from unsavory types, and as the new season gets into gear, the decisions she made at the end of the first season have come back to haunt her (I’m going to be vague about Season One, which I reviewed, in hopes that some readers will go to Hulu and devour those six episodes; it would be an excellent weekend pursuit, should your taste run to trim, energetic little thrillers that go to dark places without ever veering into uninspired grimness).

Mary’s biggest foe this season is a driven entrepreneur played by Rachelle Lefevre, and it’s interesting to see Mary go up against a woman who, under the right circumstances, can even more cold and commanding as Mary is at her most decisive. Though usually quite collected and bold, Mary’s not incapable of fear, which she begins to feel regularly this season.

Mary’s older daughter has even more suspicions about her mom this season, but the domestic side of the good doctor’s life has never been the most intriguing part of the show, and that part of the show drags at times this year. (“Mary Kills People” tends to overestimate how much I care about the protagonist’s daughter and her high school friends and frienemies.)

Many of the most rewarding scenes involve Mary alternately bickering and commiserating with the sharp-tongued Des. Dhavernas and Short have a rewarding rapport, but the characters’ relationship is tested when they temporarily split up to to continue their underground work independently. That short-lived professional separation ends up proving that they are better as a team, though whether the duo can survive the choices they’ve made is an open question. Especially given that the cops, who looked into Mary’s pursuits last season, never truly gave up the idea that she was up to something.

The other high points of “Mary Kills People” are, as was the cast last season, the meetings with patients who are nearing the end of their lives. There’s never anything maudlin about these scenes; the dying people on this show remain flawed, funny and textured until they take their last breaths. In those moments, the purity of what Mary and Des are doing is often underlined; they truly believe, not without cause, that they are doing something good by ending the suffering of those with no real treatment options as death approaches.

But the viewer must face an unfortunate fact, one the show doesn’t gloss over: We want to like a woman who is willing to do many more cold-blooded and deadly things than normal, and as the season progresses, Mary herself has to confront her own culpability and capacity for violence. Who Mary and Des are willing to kill and under what circumstances they’ll put themselves and others in danger involves a moral mathematics that continues to evolve in Season Two. At some point, they must decide where they draw the line — but only if it’s not too late to do that.

For more on the second season of “Mary Kills People,” check out this feature from Variety’s Danielle Turchiano.