Snapped was a Dark Fic that was made by the deviantART user Puffed Warrior. In the narrative, Isabella is finally fed up with Phineas repeatedly ignoring her and decides to take her bloody revenge on Ferb. The fan fic originally had its own website, but it appears to have been taken down by the user. However, you can visit her page on deviantArt to see her gallery.
This fanfic provides examples of: