Predator franchises were purchased by Marvel Comics, they had their fair share of crossovers with the DC Universe. In 2007, they took on the World’s Finest together in Superman and Batman versus Aliens and Predator.
The Predators in Superman and Batman versus Aliens and Predator were an ancient race of the Predators carrying a collection of Alien Xenomorphs as their cargo. After crash-landing in a dormant volcano, the Predators made it their clandestine home. After a group of mountain climbers were brutally slain near the site, Batman and Superman began investigating. Batman eventually ingratiates himself with the Predator tribe, besting one of their warriors and earning their respect enough that they trust Superman to take them to the Fortress of Solitude before the volcano erupts. But when Superman insists on saving the Xenomorphs too, Batman is less understanding.
In Superman and Batman versus Aliens and Predator #2, written by Mark Schultz with art by Ariel Olivetti, Superman uses what little time he has before the volcano erupts to save a Xenomorph queen and her eggs. Batman is confounded by this decision: unlike the Predators, the Xenomorphs are mindless killing machines that only present a threat to all other lifeforms. Batman vowed to never kill another human being, Superman however, has vowed to never intentionally take any life “no matter how repellent or dangerous.” The Xenomorphs are a part of the Predator hunting culture, and Superman sees to it that they leave Earth with that part of their culture intact. In only two issues, Schultz and Olivetti completely subverted superhero crossover beat-em-up expectations through strict observance of Batman and Superman’s very different “no kill” rules.
It’s worth noting that, while not a direct continuation of previous crossovers, Superman and Batman versus Aliens and Predator openly acknowledges that Batman and Superman have encountered both of these extraterrestrial movie monsters before. True to his code, Superman refuses to use lethal force against the Xenomorphs in 1995’s Superman/Aliens, regardless of how many casualties that choice led to. In Batman's Xenomorph run-ins, 1997 and 2002's Batman/Aliens volumes 1 and 2, the Caped Crusader blows up as many aliens as he can without hesitation. Superman and Batman versus Aliens and Predator brilliantly uses these different approaches to throw a wrench in the gears of what would otherwise be a straightforward, guilt-free gore-fest.
Bruce Wayne is a mortal man, his decision to never kill another human being is an extension of his crusade against crime as a result of witnessing his parents' murder. Superman’s vow against killing is largely unrelated to his birth parents' deaths, it’s more a result of the strong moral values and sense of responsibility instilled in him by his adoptive parents. Superman’s unwillingness to kill, whether it’s practical or not, comes from a pure place, a genuine love and concern for all life, even the murderous Predators. Batman, conversely, refuses to kill out of resistance, the ever present fear of who he becomes if he gives into that urge, and
Alien's Xenomorphs are an opportunity for him to get away with something he couldn’t justify otherwise.
