It could be related to Tsukishima's own past traumas or grudges. Perhaps he has some unresolved issues that make him see Ichigo as a target. He might think that by stabbing Ichigo, he can get revenge for something that he believes Ichigo is associated with, even if it's a misunderstanding.
One possible reason could be that Tsukishima is under some sort of mind control or influence from a more powerful and malevolent force. Maybe he has been deceived into believing that Ichigo is a threat to something he holds dear.
In the context of the fanfic, it could also be part of a complex plot device. The author might be using this event to drive the story forward, create drama and conflict. Tsukishima stabbing Ichigo could be a catalyst for other characters to get involved, for alliances to be formed or broken, and for Ichigo to go through some sort of character development journey as he recovers and tries to figure out why Tsukishima attacked him.
Ichigo might initially be in shock. He could be unable to believe that Tsukishima, who might have been a friend or at least an acquaintance, has turned on him.
Well, one possible plot twist could be that Tsukishima's brainwashing of Ichigo is part of a larger plan by a hidden villain. Maybe this villain wants to use Ichigo's power for some evil purpose. Tsukishima might be forced to do this brainwashing against his will in some way.
Rukia would probably be very suspicious right away. She knows Ichigo well and would notice the strange changes in his behavior. She might try to confront him and figure out what's wrong.
One possible reason could be to gain more power. In the Bleach universe, the dead often possess unique powers or knowledge. By raising the dead, Ichigo might be seeking to learn from them or use their power to fight stronger enemies.
Ichigo would probably be devastated. His trust in relationships would be severely shaken. He might become more withdrawn and focus even more on his battles and training, using it as an escape from the pain of the betrayal. He could also become more cynical about love and people in general.
One possible plot is that Chappy accidentally gets lost in the human world and Ichigo has to find her. Along the way, he discovers some new secrets about the Soul Society or his own powers. Another plot could be that Chappy gets kidnapped by an enemy, and Ichigo embarks on a dangerous rescue mission, facing powerful foes he has never encountered before.
Love and passion are common themes. Since it's a lemon fanfic, there's often a focus on the physical and emotional connection between the characters.
There's a fanfic called 'The Bond of Hearts'. It's set in an alternate universe where Ichigo and Orihime are childhood friends. It shows their relationship evolving over the years. It delves deep into their emotions and how they gradually realize their love for each other. The character development in this fanfic is quite engaging.
A possible plot development could be Ichigo being made to fight against his former allies. Under the brainwashing, he might believe that his friends are actually his enemies. Another development could be Xcution trying to extract Ichigo's power while he's in this brainwashed state.
Maybe Ichigo's death occurs due to a curse that he unknowingly activated. In the fanfic, this could lead to a whole new set of rules in the Bleach universe. His friends might then have to search for a way to break the curse and resurrect him. It could involve traveling to unknown realms or making deals with mysterious entities.