Well, if Izuku gets the Hero Killer's quirk in fanfiction, he might face a lot of internal conflict. On one hand, his original goal of being a hero is all about saving people with a smile. But the Hero Killer's quirk is more about punishing those who are 'false heroes'. Izuku would have to struggle with how to use this new power in a way that aligns with his values. Maybe he'd try to reform the quirk's nature, using it to expose real villains who are hiding among the heroes instead of attacking heroes directly.
If Izuku got the Hero Killer's quirk, it could change his relationships significantly. His friends might be a bit wary of him at first. For example, Bakugo could see this new quirk as a sign of Izuku becoming more 'edgy' and might even be more competitive or hostile. However, some like Ochaco might try to understand Izuku's situation and help him come to terms with this new power. And in the hero community, he'd likely be under a lot of scrutiny. Some heroes might see him as a potential threat, while others might hope he can use the quirk for good.