It's possible that in the fanfic, Hiccup is under a spell or has a false perception that makes him believe slaying Toothless is necessary for some greater good. But of course, it's all up to the imagination of the fanfic author.
The fanfic often explores their deep bond and adventures. You'll see them facing new challenges together and growing even closer.
You could start by imagining new adventures for them. Maybe they discover a hidden land or face a new enemy together.
In many fanfictions, Toothless and Hiccup's relationship is explored through adventure and deepening emotional bonds. Sometimes, they face new challenges together that strengthen their connection.
You could focus on developing their relationship further. Maybe explore how they deal with growing up and the challenges that come with it. Also, think about adding some unique elements to the world they live in.
To write such a fanfiction, focus on building the chemistry between Hiccup and Toothless. Come up with unexpected situations that challenge their relationship and force them to grow closer. And don't forget to sprinkle in some emotional moments to make it really touching.
Toothless might use his firepower and speed to fend off enemies threatening Hiccup. He could also use his intelligence to anticipate danger and keep Hiccup safe.
Just start by imagining unique adventures and interactions for them. Focus on their bond and the challenges they face together.
Toothless might use his speed and agility to rescue Hiccup from a dangerous situation, like a sudden attack by another dragon.
Maybe it's because Hiccup saw the goodness in Toothless and formed a deep bond that prevented him from taking such an action.
In this fanfiction, Hiccup and female Toothless usually have adventures and develop a deep bond through facing challenges together.