You can start by looking on Archive of Our Own (AO3). It has a large collection of Teen Wolf fanfictions, and you're likely to find many Stiles knows the Alpha Pack ones there. Just use the search bar and type in relevant keywords.
Fanfiction.net is also a good place to search. There are a lot of Teen Wolf fanfictions on it. You might need to do a more detailed search, like adding specific tags related to the Alpha Pack and Stiles' knowledge of them. But it can be a great source for finding such fanfictions.
Well, 'Stiles' Insight into the Alpha Pack' is also a notable one. Stiles' knowledge of the Alpha Pack in this fanfiction makes him a target for some other forces in the supernatural realm. It delves deep into his character as he tries to protect his friends while using his knowledge of the pack to his advantage. The story has a great mix of mystery, drama, and a touch of humor.
Scott would be the most likely one. He's Stiles' best friend and the True Alpha. He would never let the Alpha pack harm Stiles and would go to great lengths to rescue him.
Perhaps Stiles discovers some dark secret about the alpha pack that endangers the entire town or the supernatural world he's a part of. In his sense of responsibility and his unique way of thinking, he figures that killing the alpha pack is the only solution to prevent a greater catastrophe. This can add a lot of depth to the fanfiction story, exploring Stiles' moral compass and his ability to make tough decisions.
Perhaps in this fanfiction, the Alpha pack kidnaps Stiles because they see him as a key to something bigger. Stiles is a very intelligent character, and they might want to use his brainpower for their own evil plans. He could be held in some sort of dark and spooky lair. His friends would then have to figure out where he is, and in the process, they might discover new things about the Alpha pack and their motives. There could also be some internal conflict within the Alpha pack regarding what to do with Stiles.
Well, perhaps Stiles gets used as a bargaining chip against his friends. The alpha pack might want something from Scott or the others, and they think Stiles is an easy target to control them.
Finding specific 'Stiles kills the alpha pack' fanfiction can be a bit tricky. Some Tumblr blogs might also feature such stories or links to them. You can search relevant tags like #Stiles #AlphaPack #Fanfiction. Also, Wattpad could potentially have some, but AO3 is more likely to have a diverse range of well - tagged stories.
Well, aside from Stiles, Scott is bound to be a central character. Their friendship is so strong that Scott would immediately start looking for ways to rescue Stiles. Derek Hale is another likely character. Given his past with the alpha pack and his relationship with Scott and Stiles, he would have a role to play. Then there's Allison. Her family's history with the supernatural means she has skills and knowledge that could be useful in rescuing Stiles. She could be involved in the search and rescue operation, using her archery skills and knowledge of the supernatural world to help out.
Perhaps he feels that the other pack can offer him more opportunities for growth. In some fanfictions, the new pack might have access to different kinds of magic or knowledge that Stiles is eager to learn. So, he joins them to expand his skills and understanding.
His father could be one of the supporters. His father has always been there for Stiles, and he would understand if Stiles was standing up for what's right within the pack. Another possibility is Lydia. She's a smart and powerful character herself, and she might see the value in Stiles' actions and support him.
One way is for the pack members to stand up for Stiles more often. If someone outside the pack is giving him a hard time, they should be there to protect him. Also, they could share their secrets with him, which would show that they trust him. This trust is an important part of any relationship and would help Stiles realize that they don't actually hate him.