15He killed Wonder Woman
DC’s New 52 reboot quickly established that Darkseid’s forces were present throughout the Multiverse, with Steppenwolf—his primary general—launching an invasion of Earth-2. Steppenwolf himself killed Batman and Superman. However, the most shocking moment came during his battle with Wonder Woman, where Diana, who has fought more than her
fair share of gods, was killed by Steppenwolf in the opening pages of Earth-2’s first issue.
The Amazonian princess was fighting back hard against the general, who used a Boom Tube to sneak up behind her and stab her through the back. For a character who has never been more than just a sidenote, having Steppenwolf wipe out the powerful trio of Batman, Superman, and Wonder Woman—DC's holy trinity—gave him instant credibility as a major threat. Granted, the Earth-2 characters are the New Coke of the DCU headliners, but it’s a start.