Of all the wild crossovers for Superman over the years, one of his craziest pits the Man of Steel against Alien's xenomorphs. Superman vs. Aliens - a joint effort of DC Comics and Dark Horse Comics - tested the hero's mettle against the extraterrestrial menaces. However, his "no kill" rule didn't do him any favors.
Releasing in 1995, this crossover placed Superman in space thanks to a message that appears Kryptonian in nature. Alongside LexCorp, he goes to uncover the mysteries of the message - and finds himself engaged in battle with more fearsome foes than he could've anticipated. Like Batman, Superman has implemented a "no kill" rule and in this series, it may not be one of his best ideas. In fact, it creates more problems than benefits since his foe has no mercy.
Superman vs. Aliens, also known as Superman/Aliens, has the creative team of Dan Jurgens, Kevin Nowlan, Gregory Wright, Android Images, and Bill Oakley. Superman has trauma from killing three Kryptonians in a pocket universe. He reflects upon the experience before his eventual encounter with the xenomorphs. Despite learning how deadly the creatures are, he still refuses to employ deadly force, which ultimately has disastrous consequences and could have had even more were it not for the presence of LexCorp management.
Despite finding a colony of people trapped by xenomorphs, Superman continually tries to incapacitate the creatures with his weakening powers. He is far from the sun and the strength of his powers isn't what it usually is. So not only does he refuse to kill a deadly foe that wreaks destruction across the universe, he also does so when he's far less capable of quickly eliminating the threat. While his trauma and reasoning are very legitimate and understandable, xenomorphs are not the kind of aliens that are easy to take down. Superman makes surviving harder and even dooms others to death because of his inability to act lethally when fighting them. Thanks to his failure to destroy the xenomorphs, he becomes infected and is one of only two survivors - and he doesn't know that the teenage girl he was working with survived. To his knowledge, everyone died. He could've prevented many of those deaths had he not hesitated to end his foes by any means he had.
If the death of most of these initial disaster survivors wasn't enough, one xenomorph ends up back at Earth facing Lois. However, Superman isn't the one who saves her. Instead, it's a project leader from LexCorp who destroys the threat and effectively saves the day. Despite the initial plans to try to contain and weaponize the xenomorph, like Weyland-Yutani frequently attempts, this LexCorp employee knows the risk is too great and ends its life when the chance arises. This is a shocking twist of events since Superman is usually the hero and while he does end the xenomorphs on the colony once everyone is dead, he could've saved more people by taking the xenomorph more seriously. Not all threats can be neutralized without lethal force. While employing a "no kill" rule can be a great way to limit Superman's ability to act in many stories, him using it in the Alien crossover was one of his worst decisions. Instead of being a true galactic hero, Superman let killer aliens run amok and kill nearly everyone in their path.