The trailers for The Flash have confirmed General Zod's appearance in the movie will be more than just a simple cameo, and the reason for his return has also been teased. The primary antagonist in 2013's Man of Steel, General Zod (Michael Shannon) sought to terraform Earth into a New Krypton. However, he was ultimately killed by Superman who had no choice in the name of saving innocents. As such, it's understandable if there's confusion as to why Zod's alive in The Flash trailers.
As revealed in the trailers and synopses for The Flash, Barry Allen (Ezra Miller) will attempt to rewrite reality. This in turn will inadvertently result in a massive collision of worlds with the whole of reality being damaged. Working together with another version of himself, a Batman he's unfamiliar with (Michael Keaton), and Supergirl (Sasha Calle), The Flash will attempt to restore the universe and set things right. However, it looks as though his primary foe will be none other than General Zod.
Zod's Return As Explained By The Flash Trailers
In both trailers for The Flash, Barry Allen's confessions about breaking the universe are closely followed by shots of General Zod and his Kryptonian forces. He's also featured using the same terraforming machines he used in Man of Steel. This implies that Zod's original campaign will still be in effect in this warped reality, and it's likely happening due to the absence of Superman (who's heavily teased as being dead in this new timeline). This would also explain Batman and the Flashes needing Supergirl's help.
Based on the framing of The Flash trailers, Zod's return seems to be directly tied to The Flash's actions and the collision of realities. Somehow, this version of Zod likely didn't face the same obstacles (such as Henry Cavill's Superman). Instead, events have likely been altered which ultimately served the Kryptonian villain better in comparison to the original events seen in Man of Steel.
How Barry Allen Going Back In Time Leads To Zod's Return
As it stands, it's almost a certainty that Barry Allen attempting to save his mother in the past will create a cascade of unfortunate events that leads to no Superman and subsequently General Zod's arrival on Earth virtually unopposed. The domino effect will begin with Barry in a very similar fashion to the original DC Comics Flashpoint storyline. As such, it looks as though Barry and his new allies will have quite the battle ahead of them to restore the timeline when The Flash releases in theaters on June 16th.
