Well, a number of fans are divided. Some feel that the insults are a form of character assassination for Sasuke, as they view him as a more complex and redeemable character than one who would just randomly insult Sakura. Others, though, might see it as a way to explore the darker side of their relationship and how Sakura can grow from being insulted and become an even stronger character.
Sakura would be completely devastated. She's been in love with Sasuke for so long and has a deep bond with Naruto too. She might go into a deep depression and lose her will to be a ninja for a while.
Sasuke would probably be very guarded at first. He's not used to dealing with something so innocent and vulnerable. He might be suspicious of the baby's presence and think it's some sort of trap.
One element is the lyrics. If the lyrics are about dark topics like death, ghosts, or curses, it can be scary. For example, lyrics that describe a spooky encounter with a supernatural being. Another is the tone of the rap. A slow, deep, and menacing tone can add to the scariness. Also, the setting where the rap takes place. If it's in an abandoned place like an old factory or a haunted house, it gives a creepy vibe.
Generally, fans have a mixed reaction. A large portion of fans who support the Sasusaku relationship feel betrayed or disgusted by such fanfictions. They feel that the characters have been misrepresented. On the other hand, a small group of fans might view it as a way to explore the limits of the characters and the Naruto universe. But overall, it is a very controversial topic within the Naruto fan community, with more fans leaning towards not accepting this kind of extreme deviation from the original story's character arcs.
Some fans are really into it. They enjoy seeing different takes on the Naruto and Sasuke relationship. It gives a whole new dimension to their dynamic. However, some fans might be a bit put off as it goes against the more traditional view of their relationship in the original anime and manga.
Fans have a wide range of reactions. There are those who are vehemently against it. They have a strong emotional investment in the Sasuke - Sakura relationship from the original series. Then there are others who are more open - minded. They see it as an interesting exploration of different relationship scenarios within the fanfiction world. These fans might engage in discussions about why the author chose to take such a route and what it means for the characters' development in the fanfic.
Sasuke would be furious. He's always been protective of Naruto in his own way. He would immediately start looking for her, using all his skills and resources. He might even go on a rampage if he thought Naruto was in serious danger.
On the other hand, Sasuke could be very stoic about it on the surface but internally be in turmoil. He might start to analyze his own actions and whether he had missed any signs from Sakura. He could also be the type to immediately start thinking of ways to win Sakura back if he realizes he can't stand the thought of her with someone else.