Maybe John was frustrated at Sherlock's eccentric behavior. Sherlock often acts in ways that are outside of normal social norms, like his extreme focus on cases and disregard for basic social niceties. This could have led John to lash out and call him a 'freak' in a moment of exasperation.
He might be initially shocked. Sherlock is used to John being more understanding, so this outburst could catch him off - guard. But then he could go into his logical mode and start analyzing why John would say such a thing.
In many Sherlock fan fictions, Sherlock branding John could be a sign of his trust or a way to get a reaction out of him. It all depends on the author's imagination and the context they create.
In many Sherlock/John fanfictions, Sherlock's actions towards John show deep affection. For example, he often goes out of his way to protect John from danger. He might put himself in harm's way just to keep John safe, which can be seen as a sign of love. Also, his jealousy when John pays attention to others can be an indication. In some fanfictions, Sherlock is overly possessive of John's time and attention, much like a person in love would be.
The strong bond between them. Their relationship is a mix of friendship, mutual respect and a bit of mystery. In fanfics, writers can explore this relationship further, like showing how they rely on each other in different situations.
There could be a 'baby sherlock daddy john fanfic' that focuses on the emotional bond between them. For instance, in the story, John might be worried about Sherlock's future because of his different way of thinking even as a baby. It could also explore how Sherlock, despite being a baby, forms a special attachment to John. This kind of fic would be really heart - warming as it delves into their relationship at such an early stage.
Often, these fanfictions focus on the slow - burn aspect. Since Sherlock and John's relationship in the original series is platonic - ish at first, the fanfics gradually build up to the kiss. They also tend to explore the characters' emotions more deeply, like Sherlock's struggle to understand his own feelings.
If Sherlock was John's kid, it could change the nature of their relationship. There might be more protectiveness from John and a sense of inheritance in Sherlock's detective skills.
If Sherlock was John's kid, it would likely bring a unique emotional depth to the story. It might force them to confront personal issues while still trying to crack cases. It could be a fresh and exciting take on the usual detective duo.