[This story contains spoilers from the series finale of FX's The Strain.]

After four seasons of sacrificial deaths and the biggest infestation of worms television has ever seen, FX’s The Strain came to a close Sunday with a series finale that closed the door for good on Eph's (Corey Stoll), Zach's (Max Charles), Fet's (Kevin Durand) and Dutch’s (Ruta Gedmintas) ultimate fight against the Strigoi.

Following Setrakian's (David Bradley) death, the resistance fighters teamed in one last bid to take down The Master (Jonathan Hyde), as led by Mr. Quinlan (Rupert Penry-Jones) and the giant nuclear bomb Fet and company snuck into the city. In the end, The Master took out his son, Quinlan, but not before walking into an underground trap with Zach where the nuke was waiting. Although Fet had every intention of sacrificing himself and detonating the thing, it was Eph who launched himself down to the basement and faced off one final time against The Master — just as Zach realized he had been on the wrong side all along.

Although Zach shot at The Master and tried to take him out, in the end The Master transferred to Eph’s body in a gory rainfall of oozing worms. It was the last straw Zach needed to take the lead Strigoi out himself as he apologized to his father for what he had done, said “I love you” and then detonated the bomb.

Below, showrunner Carlton Cuse — who co-wrote the finale with co-creator Chuck Hogan — talks with THR about ending The Strain with a bang and what he learned from other series enders.

Was there ever a version of this finale where everybody just died in a glorious blowout?

We felt like there had to be some glimmer of hope at the end. We did actually discuss bleaker versions, but it just didn’t seem like that would be very satisfying. The audience would feel too depressed. It was too hard to contemplate not having some sort of consequential victory at the end of the series.

Was the plan to always have Zach come full circle and detonate that bomb in the end?

We embraced this idea of making this kid truly a villain, and that’s something we felt we hadn’t seen before. Yet it was fair to have him have just enough of a self-realization at the end to decide to participate in The Master’s demise. The audience was waiting for us to do some sort of massive redemption for the character and it didn’t feel that that would be fair given how villainous the character was. But this was a little bit of an unexpected twist and a bit of a more believable and earned turn for the character.

When was that conceptualized, and did recasting the role with Max Charles in the second season have anything to do with it?

We had a lot of conversations about it; it’s fair to say you can’t really construct a finale in the abstract way ahead of time. Everything is informed by the creative journey that takes you to the end. Over the last two years of the show we had a lot of conversations about what the ending would look like — it moved around a little bit. We eventually got a place where we felt like we’d made the right choices. I’m not sure people took me at my word when I said back then that we had expectations that the character was going to be a darker version, and that it was really not about the actor, it was about finding a different type of actor who could go to a much darker place. I think we proved that was true. No one expects kids or grandparents to be villainous. It’s like storytelling convention 101; those characters are supposed to be good or ultimately good. We really wanted to change that up and make Zach a horrible antagonist. Max embraced it and did a great job of bringing that character to life. With Setrakian, we also tried to avoid having him be the sentimental grandfather figure. In a lot of movies those characters play mentor figures and guides who are wise. Ultimately Setrakian was revered by his compatriots, but he certainly was not sentimental or kindly in the traditional way we see those characters.

As a fan favorite, was Fet’s survival inevitable?

We love Fet. Ultimately it felt that his survival was biblical. The first shall be last and the last shall be first. Here’s this guy who was a rat catcher, whose status was low in the world before this epidemiological crisis and yet somehow when this convulsive event strikes he’s almost the best-equipped person around to not only survive, but to help other people survive. For all that he gave he deserved to live and to win in the end.

Usually heavily serialized shows like this result in talks of spinoffs or movies; were you purposefully closing all doors to that kind of talk here by resolving all of the characters’ stories?

We never spoke of any spinoffs; we wanted this to be conclusive. And I guess in being conclusive you shut down many of the avenues for sequels. It felt like the story had an ending that was fully resolved. The Master had to be defeated at the end of the journey and once he was defeated that was kind of the end of the threat.

You’ve probably crafted more planned series finales than most showrunners, having recently wrapped A&E's Bates Motel, too. What have you learned about that process that you wanted to convey to the rest of the team in ending this?

Particularly now in the streaming environment where shows have a life well beyond their traditional linear broadcast life, endings are really important. People watch shows and invest in them hoping they’re going to end well or at least end conclusively. So for the history and legacy of The Strain we had to define what the journey was. We had originally sold the show as being three seasons and added a fourth along the way. At one point there was talk of it being five seasons, but it really felt that four seasons was right. We wanted to bring it to an end while we still had the appropriate amount of narrative velocity going.

What I’ve learned about doing finales is that you have to try to think about your story at a 30,000-foot level and decide what kind of ending the show deserves and what kind of an ending you can craft given the type of story you’re telling. Obviously there’s controversy about the Lost ending, but I promise there was no version of the Lost ending that was about answering the unanswered questions of the show that wouldn’t have been didactic and much worse than the ending that Damon Lindelof and I crafted. Kerry Ehrin [who was the co-creator and co-showrunner on Bates with Cuse] and I found our way to a beautiful ending with Bates Motel and it was something we had thought about for a long time — a Romeo and Juliet ending that was appropriate to the romantic tragedy of the show. And with The Strain, this epic graphic novel thriller-adventure, it needed to end with The Master being vanquished. But given the tone of the show and the way the story played out it felt that it needed to be played out at a high cost and a lot of characters had to be sacrificed for that goal.

Given that there were talks of five seasons at one point, was there anything left on the cutting room floor that you had really wanted to incorporate?

In the last season of a show you can really do anything; we wanted to not have everything finish at the end, which is why Setrakian’s story wraps in [episodes] seven and eight. So we spent all our bullets, we told the story we wanted to tell and we’re happy about it.

Looking toward the future, what is the status of the Jack Ryan series that you're doing for Amazon?

Jack Ryan will be on sometime during 2018; it’s going great but it’s a huge and complicated show with a lot of visual effects and elaborate postproduction. It’s really good but it’s going to take us until sometime early next year just to finish all our work on the show. And then Amazon has to decide when they want to put it on.

Has the lawsuit with Paramount affected anything in the meantime?

That hasn’t had any effect on the production of the show.

You’ve also got a huge overall deal with ABC Studios. Is there anything you're already working on that you can discuss?

I’m really excited about the new deal; I love everybody at ABC Studios, ABC and Disney but the deal is really just starting so it’s too early to talk about the specific projects. There’s going to be some cool opportunities that I’m really excited about getting going on though.

Do you have a preference between building out shows based on popular source material like Bates Motel, The Returned and The Strain or original series like Colony and Nash Bridges?

It’s really just all about good ideas, whether it’s finding a good idea from an existing IP or that a writer has or something that pops into my own brain. I don’t have any particular prejudice to one over any of the others. I love, love, love storytelling and it’s just about finding good stories.

Thoughts? Sound off below.