WARNING: The following contains spoilers for Dark Knights of Steel #2, on sale now from DC Comics.
The world of Dark Knights of Steel isn't one of traditional heroes and villains. There are a lot of grey areas, with these mythical interpretations of DC's heroes coming into conflict for a number of reasons, none of which paint them as truly good or truly evil. However, that changed when the series introduced its version of Supergirl.
The first issue ended with this world's Green Arrow making an impossible shot that killed Jor-El. The consequences of this shocking death played out in Dark Knights of Steel #2 by Tom Taylor, Yasmine Putri, Arif Prianto, and Wes Abbott. Though King Jefferson Pierce hoped that the death of the Kryptonian king would prevent a prophecy of ruin, all he did was bring it closer to coming true. The Kryptonian who did this was none other than this world's Supergirl.
In the main DC Universe, Supergirl is Superman's cousin, Kara Zor-El. Here, however, she's his sister, Zala Jor-El. This would mean that she was born on this Earth, shortly after Superman and his family crash-landed there. When she first appeared in this issue, she was training on Amazon Island with her lover, Wonder Woman. there was very little was revealed about her before she learned of her father's death. Therefore, the only way to judge what she's really like and how she compares to Kara is through her reaction to that death and, unfortunately, it's pretty bleak.
Her first response to finding out that King Pierce ordered the death of her father was to fly right to his kingdom and kill his young son. When the boy threatened to make it rain in her lungs she wasn't afraid of him. She simply dropped him and let him fall to his death. She didn't even fear the mighty king. When he used his lightning powers on her, it had no effect. She even claimed it tickled. Green Arrow was taking down someone he knew was far more powerful than him. Supergirl, on the other hand, killed a defenseless child.
The issue even made a point of noting that the Dark Knights of Steel Superman isn't a killer, even when filled with anger over his father's death and confronting his killer. This makes it even more shocking when Zala committed murder for the same reason.
It's clear that this version of Supergirl lets her anger get the better of her, and in the worst way possible. The Supergirl of the regular DCU has actually been through something similar, with some destructive consequences too. When she became a Red Lantern in the New 52, the red ring drove her into a rage that sent her on a rampage. She almost killed her best friend, but was able to resist the urge and found help. She eventually learned to control the immense anger that was pent up inside of her with the help of Guy Gardener.
Why was Supergirl angry enough to inherit a Red Lantern ring though? Well, because she had lost everything. Her homeworld was destroyed and everything in her life since that moment had gone horribly wrong. Although Zala just lost her father, that's nothing compared to Kara's loss. It's important to note that Kara resisted the insatiable Red Lantern urge to kill, whereas Zala didn't even try to resist her own. Instead, she decided to hit her enemy where it hurt and in the cruelest way possible.
Zala Jor-El could be the multiverse's most violent Supergirl. The fact that she is unable to control her anger here, and the fact that's she's been trained to kill by the Amazons as well, makes her extremely dangerous. This cruel murder could very well be just the tip of the iceberg. Who knows what else she's capable of?