What happens if you found yourself facing a child-version of what you knew would become a deadly threat to the world? This isn't a new question. Often, the question is tied to someone like Hitler and would it be right to go back in time and kill him as an infant?
This is a similar situation to what Fantomex dealt with back in UNCANNY X-FORCE #4. While on a mission to destroy Apocalypse before he could gain full power and proceed to wreak havoc and destruction upon the world, the team found themselves facing what appeared to be an innocent child being groomed by a genocidal cult to become the evil despot.
When the moment came to deal with the child, there was some hesitation. It was Fantomex that decided to pull the trigger. Now he finds himself on trial by the Captain Britain Corps in the mythical realm of Otherworld for his actions.== TEASER ==
We know killing shouldn't be the solution for heroes but Fantomex made the decision based on the old argument. He figured that destroying what would become pure evil was the best way to deal with the problem. It's really a matter of whether or not the child should be held responsible for actions they have yet to commit.
In the case of killing Baby Hitler, it's already known what that infant will grow up to do. Going into the past, the future has already happened. There's no question whether or not the 'evil' person will stick to their evil path.
In the case of Fantomex's decision, the child was being groomed to become Apocalypse. He didn't know who he was or who he was supposed to be. He was being educated to become a version of the Apocalypse we all know. Could there have been another solution?
Fantomex made another decision after killing the child. He had a clone created who was raised in a virtual reality world. Killing a child wasn't a decision Fantomex took lightly and by making a copy of the child, he could see if there was a chance to redeem that child. This would be a way to explore the argument of Nature vs. Nurture.
Is the evil contained in a villain due to their genetic make up or does it have to do with how they were raised? The child that Fantomex killed was being programmed to follow the ways and thinking of Apocalypse. With Genesis, there is the chance to see if being raised in a loving and caring home could change what appeared to be the child's future. In other words, was he born evil or simply raised to be that way?
Another example of Nature vs. Nurture is Spider-Man and Kaine. Peter Parker was raised by his loving aunt and uncle. The idea of great power and great responsibility was ingrained into his way of thinking. This wasn't the case for Kaine. He was created in a lab and the only thing he had to a parent (the Jackal) cast him out in utter disappointment for being defective. It's no wonder Kaine's way of thinking was muddled yet now, he's finding himself acting more like a hero. He doesn't want to go out and actually become a superhero but he's finding it harder and harder to deny the urge to do the right thing.
There are simply too many factors that go into what makes a person good or evil. Most comic book universes don't allow an individual to travel back in time to alter the past. It usually leads to further complications or the creation of an alternate timeline/reality.
If the future was known, it's a little different but the argument could still be made that the future is never set in stone. Knowing that a child could grow up and become a deadly force such as Apocalypse doesn't guarantee that they will. If there is some remote trace of goodness in the child, the proper upbringing might be able to turn things around so he'd become a savior instead.
Of course there's also the notion that heroes don't kill. I think even the noblest of heroes would have a hard time taking a life, even if it would guarantee saving the lives of numerous others, especially when that life is at an innocent stage.
There may not be a clear cut solution. Fantomex made a decision and is now dealing with the consequences. Whether or not Genesis will grow into a hero or still become an evil force (as seen in a possible future) remains to be seen. If a hero was ever placed in the situation where they had to make this decision, you can imagine they would struggle with making a decision.