He might also be confused. Kiba knows Naruto as a friend, so seeing Naruto hold Akamaru hostage would be completely unexpected. He may try to talk to Naruto first, asking him what's going on and trying to reason with him, while also being on edge and ready to attack if necessary.
One possible motive could be that Naruto is being controlled by some dark force or genjutsu. Maybe an enemy has manipulated his mind to target Akamaru in order to get to Kiba. Another reason could be a misunderstanding. Naruto might think Akamaru has been infected with something dangerous and he's trying to keep it away from others by holding it hostage, but in a very wrong way.
Naruto would probably be shocked at first. He might not have expected Hinata to leave him for Kiba. He could be angry at himself for not noticing Hinata's unhappiness sooner.
One great 'Naruto Kiba x Akamaru' fanfic is 'Bond of Fang and Master'. It beautifully portrays their relationship. It shows how Kiba and Akamaru grow together, their training sessions and how they rely on each other during missions. The story is filled with action scenes that highlight their teamwork.
The popularity might also stem from the fact that Naruto has a huge fan base. With so many fans, there are bound to be those who are interested in exploring different pairings in a yaoi context. Kiba and Akamaru have their own charm and personalities in the series, and fans like to build on that in fanfiction. They can add new storylines and emotions that weren't fully explored in the original anime or manga. For example, exploring how their relationship could develop in different scenarios or how they might deal with new challenges together in a more romantic or intimate way.
Shikamaru would likely be lazy about getting involved at first. But if it started to cause a big ruckus in the village or among their group of friends, he'd step in. He'd probably analyze the situation quickly and try to find a logical solution. He might tell Naruto to cool off and Kiba to stop provoking Naruto if that was the case.
Well, I'm not sure about specific 'Naruto Kiba Akamaru Yaoi Fanfiction' stories. But you can try searching on fanfiction websites like Archive of Our Own. There are usually a lot of different fanfictions there, and you might find some interesting ones about this pairing.
He could discover it during a dangerous mission in an ancient ruin. Maybe he was exploring an old ninja stronghold and came across the sword hidden away. It might have been protected by traps and guardians, which he had to overcome to obtain it.
One way is through shared adventures. For example, they could go on a mission together where they face a really tough enemy. This would make them rely on each other more and deepen their bond. Another way is if Naruto starts training Akamaru in some new skills. This would not only make Akamaru stronger but also show Naruto's care for him.
You can try fanfiction.net. It has a large collection of 'Naruto' fanfics, and you're likely to find some good ones about Kiba and Akamaru there.
Naruto would be extremely hurt. He loves Hinata deeply and to find out she cheated with Kiba would be a huge blow to him. He might go into a state of shock and disbelief at first.