Zack Snyder's Justice League features a more sophisticated Steppenwolf than the villain's previous iteration from 2017's theatrical release, and the longer version explores a key betrayal in his past. In the Snyder cut, Steppenwolf is given enough time on screen to establish a more developed motivation for attacking earth than simply being a power-hungry bad guy. He is still very much that, merciless and otherwordly, but Snyder's Steppenwolf enjoys increased depth thanks to a well fleshed out backstory: Steppenwolf betrayed new villain Darkseid and, as punishment, was banished from Apokolips.
Betraying Darkseid is why Steppenwolf is so eager to unite the Mother Boxes and create the Unity. His only way to be allowed back home to Apokolips, and into his master's good graces, is by conquering 50,000 worlds in Darkseid's name. Much of this important information is established through dialogue between Steppenwolf and Desaad, Darkseid's second in command, key moments in the Justice League Snyder cut that construct Steppenwolf's characterization. His conversations with Desaad, and ultimately Darkseid, vastly elaborate on his purpose, which forms a much deeper connection between him and the film's plot, even establishing him as a debatably tragic figure, one who seeks redemption, but ultimately fails before tasting it.
In the DC comics, Steppenwolf, created by Jack Kirby, was one of the New Gods that lived on hellish Apokolips. His brother was the planet's first ruler, Yuga Khan, whose son was a New God named Uxas. Lusting for dominion over Apokolips, Uxas battled his father and killed him, but not before Steppenwolf, and many of the New Gods, presumably battled Uxas. Uxas won, claimed Apokolips, and took the name Darkseid. While in Snyder's Justice League, Desaad mentions that Steppenwolf is Darkseid's nephew, not uncle, there are parallels between the betrayal from the comics and in the film. In the DCEU, Steppenwolf challenged Darkseid's right to rule, and that betrayal resulted in his banishment.
Although Steppenwolf claims that he, "saw his mistake," and, "slaughtered those who sought his throne," meaning Darkseid's seat of power, it's clear that Steppenwolf had once defied Darkseid by choosing to serve himself. This backstory is similar to Steppenwolf's in the New Gods comics, as Steppenwolf would have probably fought with Yuga Khan against Uxas before the latter became Darkseid. Although Steppenwolf was a mighty addition to Darkseid's forces, there is clearly no easy way out with the Lord of Apokolips, even for a warrior so formidable as Steppenwolf, which explains his banishment.
The betrayal of a heartless warlord like Darkseid is a costly transgression. Desaad reminds Steppenwolf that he, "might have sat here, by the side of the great one" if it weren't for Steppenwolf's "self-pride". It is possible that Steppenwolf once thought, just as how in the New Gods Uxas overthrew Yuga Khan, he could take Apokolips from Darkseid. It is evident that Steppenwolf failed to show fealty, and that lack of loyalty resulted in a banishment from his homeworld that lasted millennia.
During the scene in Justice League that follows Steppenwolf's first meeting with Desaad, Wonder Woman explains the history of earth's Mother Boxes to Batman. Snyder implements a flashback here, offering a first DCEU look of Darkseid in action. An epic battle takes place, showing Darkseid and his Parademons warring against humans, Atlanteans, Amazons, Green Lanterns, and several gods from classical Greek mythology. Blatantly missing from Darkseid's failed invasion of earth is Steppenwolf and his electro-ax. This might be another hint at Steppenwolf's betrayal; that his pride, and ultimate undoing, was the fact that he served himself, not Darkseid, during the latter's first campaigns, even before Darkseid assumed rulership of Apokolips. That could be why Steppenwolf is not present during Darkseid's defeat on earth. For certain, Steppenwolf did not serve Darkseid properly, and in Zack Snyder's Justice League, he's still paying for that betrayal.
