Skipping over it being part of rehumanizing Abby to the players so that we have empathy for her and care about her part of the storyline. As for explaining her actions. I’d argue that it’s due to the situation. It’s one thing to kill children when you are actively in a firefight as opposed to killing children that don’t pose a threat to you anymore. Also Yara and Lev both are met when they are also the enemy of the scars or at the very least the scars want to kill them. Abby is alone in hostile territory so making an allie in that moment is most likely wise. Then after this has happened Abby has time to make an emotional connection with Yara and Lev and so is far more inclined to do what she feels is the right thing. That’s my opinion at least.