Andrew Kreisberg says killing off the beloved character "was literally the worst thing we could do to ourselves as writers, producers and friends."

[Warning: Spoilers ahead for Tuesday's The Flash season finale, "Fast Enough."]

A member of Team Flash's has made the ultimate sacrifice.

Eddie (Rick Cosnett) took his own life in order to stop his descendant Eobard Thawne/Reverse-Flash (Tom Cavanagh) in Tuesday's finale of The Flash, realizing if he killed himself, Eobard wouldn't be born. The plan worked, and Eobard vanished from existence after Eddie shot himself, leaving Barry (Grant Gustin), Eddie's former girlfriend Iris (Candice Patton) and the rest of the team to mourn.

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Eddie wasn't the only one to sacrifice. Earlier, Barry made the decision not to prevent his mother Nora's (Michelle Harrison) death, because doing so would prevent him from becoming The Flash and would mean that all the good he's done as the hero would be undone. It's a monumental decision, meaning Barry will go forward fully embracing his identity as a hero.

The finale leaves plenty of questions, among them: Will Eddie or Eobard be on the show in the future?

In a chat with The Hollywood Reporter, executive producer Andrew Kreisberg reveals that Cavanagh will remain on the show as a series regular, weighs in on Eddie possibly returning, and teases "a bunch more villains" as well as additional speedsters for season two.

When did you know Eddie would make this sacrifice?

When we decided to name him Thawne, we hoped the audience would suspect Eddie was the Reverse-Flash because of his last name. We always knew Eddie would be his ancestor, but we weren't quite sure how we would end the season. The way things were moving forward, it felt like it was the best thing to do for his character. Like with Colin Donnell [whose character Tommy died in Arrow's season one finale], it was literally the worst thing we could do to ourselves as writers, producers and friends, because we all love Rick so much both personally and professionally, and we think he's crushed it as Eddie all season. We've all become very close. It's one of those terrible things. The story sort of tells you what it wants to be and as much as it broke our hearts, we knew this was the way the season needed to end.

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What was Rick's last day on set like?

Usually the last day on set tends to be something very mundane. The last day on set turned out to be his death scene. It was a very complicated day, visual effects-wise, because of all the things that were going on. It was also a difficult day emotionally. Not only was it the end of the season, and everyone from the crew to the cast has just been killing themselves to make this show as good as it could be, it was also incredibly emotional. Just like Collin Donnell, Rick was told about the plan way in advance, because obviously we didn't want him to read it and be surprised. As soon as he found out, he kicked up his acting level a notch. It was almost subconsciously — his performance got even stronger, which again makes it even worse emotionally for us. Rick has been turning in his best work of the year in these last few episodes just like Colin did. I literally can't say enough about Rick Cosnett.

Will Eddie be back?

The great thing with our show — you saw it with Colin Donnell and with Caity Lotz [whose deceased Sara is returning for spinoff Legends of Tomorrow] — is just because you are dead doesn't mean you're not coming back. Especially in the world of The Flash, which involves time travel and real hardcore science fiction, there's always a way for Eddie to return, and we hope Rick will.

How does Eddie's sacrifice work? Eobard disappears — but everything he did up until the finale still happened?

Our time travel hopefully holds together as much as it can. It doesn't completely obliterate all of their memories of Eddie and everything, but it has the desired effect of "harm to Eddie means harm to Tom Cavanagh's character."

How did you lay the groundwork for Eddie to make this choice?

Eddie has been struggling these last few weeks, hearing about the future and about how there is no place for him in the future. He wasn't going to believe in Wells' interpretation of the future. He was going to make his own decision and he basically decided to recommit to Iris, which only makes his sacrifice that much more heartbreaking. He didn't do it because he didn't have anything to live for. He did it because he had everything to live for.

What does this mean for Tom Cavanagh's future on the show?

Tom Cavanagh will be back. That is not in question. Tom Cavanagh will continue to be a regular.

This season was so well planned out in advance. Is that the approach you will take with season two — constructing a big master plan like you had for season one?

Not to pat ourselves on the back too much, but if you watch the pilot, there are clues that got laid out throughout the season. We've done the same thing for season two, and hopefully we've created a structure and a scenario for season two that people will find equally compelling and equally interesting. Some of it has already been set up in the events of season one — just like we do on Arrow where, without even realizing it, toward the end of the season we're setting up the next season. Once people come back in season two, they'll look back at some of these episodes in season one and go, "Oh wait a minute, I see where this came from."

You've said season two will introduce more Speedsters. Is that going to be a major theme akin to the Rogues in this season?

Yeah. We are going to introduce a few more speedsters next year and a bunch more villains. How they and those villains come about is part of the surprise of season two. We're really excited. [Executive producer] Greg [Berlanti] and myself and [executive producer] Geoff Johns and the writers, the cast, the crew, the directors — we are so proud of this season of television. It really is a high mark for all of us, and we feel a great deal of pressure and anxiety to live up to it because it's been so well received. As proud as excited as we are about everything we've done this year, we really are just as proud and excited for all the things we are planning coming up. Hopefully people will continue to take this ride with us.

For more from the finale, check out seven Easter eggs teasing season two and Legends of Tomorrow that were included in the episode.