Sakura might be in shock at first. Unable to believe that Naruto, who has always been so strong - willed, could have ended his own life. As she processes it, she could become angry, not at Naruto but at herself and the situation that led to this tragic outcome.
Naruto could try to talk her out of it. He is a firm believer in the power of friendship and words. So, he would likely use his words to try and break whatever spell or wrong idea is in Sakura's head that made her want to kill him.
Sakura might initially be confused. She has known Naruto for a long time and his sudden coldness would be unexpected. She may try to reach out to him to find out what's wrong.
She may feel angry at first. Sakura is a strong - willed character. She might think that Naruto has made this decision without considering her feelings. In the fanfic, she could go on a rant about how he should have talked to her before making such a big decision.
In this type of fanfiction, a major theme is often the fragility of the male ego in the face of unrequited love. Naruto, despite his strong exterior, is broken by Sakura's lack of romantic interest. Another theme could be the idea of lost hope. Naruto may have hoped for so long that Sakura would come to love him, but when that hope is finally extinguished, he sees no reason to continue living. Additionally, themes of loneliness and isolation might be present as Naruto may feel completely alone in his unrequited love for Sakura.
She might be shocked at first. Sakura has always known Naruto as someone who is very attached to Konoha. If his leaving is because of her, she could feel guilty. She might try to stop him, asking him why he's doing this and telling him that he doesn't have to go.
She might be heartbroken. Sakura could initially be in shock and then fall into a deep sadness. She may blame herself for Naruto's leaving, thinking that she could have done something differently.
Some fans are horrified. They love the characters as they are in the original series and any deviation that involves such a violent act against a main character like Sakura is unacceptable to them.
She might be confused at first. Sakura is used to Naruto being friendly and now his coldness would be a big change. She may start to question what she did wrong.
She might be initially shocked. Sakura has always seen Naruto as a friend, so learning of his romantic feelings could completely change her perspective of him. It could make her re - evaluate all their past interactions.
She could be confused at first. She might not understand why Naruto is suddenly ignoring her, since they were friends before. So she might try to approach him to ask what's wrong.