The Walking Dead: World Beyond's Iris killing the CRM soldier in the season 2 premiere brings her closer to Hope. The CRM wiping out the Omaha settlement has led Iris down a path of vengeance that begins when she kills the soldier. She is now planning to completely eradicate the entire organization. On a much smaller yet incredibly personal scale, Hope went through a similar situation when she and Iris were children.
In World Beyond's season 1 premiere, there are flashbacks shown of Iris and Hope as children when the apocalypse begins. After separating from her sister and Leo, Hope and her mother Kari cross paths with a pregnant woman who, in a panic, ends up shooting and killing Kari. Hope kills the woman after the gun she picks up accidentally goes off. She later learns that the woman is Elton's mother. By the end of World Beyond season 1, Hope tells Elton the truth about his mother, which puts an obvious strain on their friendship.
Killing the CRM soldier allows her to understand why Hope killed Elton's mother, and the feelings she was likely left with in the aftermath. Before this, the incident got in the way of their relationship, creating a noticeable chasm that arguably isolates Hope even more given her simultaneous tensions with Elton. Both sharing the same experience can help strengthen their bond, and may even help Hope talk Iris out of making rash decisions that result in dire, if not fatal consequences.
At the beginning of World Beyond, Iris has an idealistic view of the world, primarily due to being raised in the Omaha settlement for the majority of her life. As such, she views the CRM as good and believes they are making the world a better place. Only after hearing out Hope's suspicions and seeing how the death of her therapist is handled does she realize the CRM is worse than she thought. This paves the way for Iris to begin seeing things from her sister's perspective, later solidified by her killing off the soldier.
As mentioned before, the truth about Hope killing Elton's mother puts a strain on the relationship she has with Iris. This is because, at that point in the series, Iris maintains a black-and-white view of the world. It is only by killing the soldier in a fit of rage that she is able to understand what Hope went through. She may now even believe she has a greater capacity for darkness, because while Hope was still a young child who accidentally killed Elton's mother, Iris is mature enough to know right from wrong yet still intentionally kills the soldier.
Iris' first human kill in The Walking Dead: World Beyond brings her closer to her sister because she is now able to empathize with her in a unique, albeit brutal way. Still, killing the soldier has not deterred her from her current path of vengeance, and her plan to wipe out the CRM could lead to many deaths. With Hope still adamant about sparing the civilians of the Civic Republic, this could bring the siblings into conflict with each other.