They would be absolutely devastated. Minato, as Naruto's father, would likely be filled with guilt for not being there to protect him. Kushina, as his mother, would be heartbroken. They might first be in a state of shock, unable to believe what has happened. Minato might blame himself for leaving Naruto alone in a world full of dangers. Kushina would probably hold Naruto's body close, crying uncontrollably, and their grief could be so overwhelming that it would change the entire course of the ninja world they live in as their connection to Naruto was a major part of their lives.
They would both feel like their world has ended. Minato would be in disbelief and would keep checking Naruto's vital signs, hoping it was some kind of mistake. Kushina would start to question everything. Why did they have to die so early? Why couldn't they protect their son? Minato would become more withdrawn, his usual confident self gone. Kushina would try to hold on to the memories of Naruto's smiles and his determination. They would both be haunted by the thought of all the things Naruto could have accomplished if he were still alive.
Minato might be intrigued. He would be interested in seeing how the fans expand on Naruto's journey. Kushina would be delighted, seeing her son as the center of so many different tales.
One possible fanfiction could be where 'evil Minato and Kushina' are under some sort of dark magic or mind control. They find Naruto not as their beloved son but as a target. Maybe they are being manipulated by a powerful enemy of the village who wants to use Naruto's power for evil. However, Naruto's pure heart and the bonds he has formed with his friends start to break through the false personas of his parents.
At first, Naruto would be shocked and confused. His world view would be shattered. But then, he would draw on his inner strength. He has overcome so many hardships before. He might feel a deep sadness, but also a sense of resolve. He would still believe in the power of love and friendship, and use that to face Kushina's betrayal. He could also use this experience to grow even stronger and more determined to prove himself and his beliefs.
Minato would probably be very proud. He might notice Naruto's use of his own jutsu and be impressed by how Naruto has developed his skills. Kushina would be more emotional, maybe crying at the sight of Naruto growing up so well.
Minato would likely be extremely shocked. Naruto is his son, and the idea of Naruto attacking him would be completely unexpected. He would probably try to talk to Naruto first, to find out what is wrong and why he is being attacked.
The story would change a great deal. For one, Naruto wouldn't have the same connection to the Nine - Tails as he does in the original. Minato sealed the Nine - Tails into Naruto, so without that relation, the whole dynamic of the Nine - Tails within Naruto would be different. He might not have the same power - up moments related to the Nine - Tails.
Well, in such a fanfic, Naruto would probably have a very different personality. He might not be as optimistic. His training might take a different path too. Instead of having the Nine - Tails as a sort of legacy from his father, he could see it as just a curse that was left with him when he was abandoned. His bond with the village might be weaker as he was rejected by the two most important people who were supposed to be from the village in the first place.
Minato would be in awe of the new ninja world. He'd notice the changes in the village's infrastructure and the way ninjas are trained. He might be a bit taken aback by the more complex political situations. However, being the quick - thinking ninja he is, he would start to analyze everything. He'd work with Naruto to understand the new power dynamics and use his old - school wisdom along with Naruto's modern knowledge to make the village even stronger.
In some cases, Naruto might initially become bitter towards his parents. But as he experiences more in life and learns about the possible reasons behind their actions, he could come to understand and even forgive them. His reaction would also depend on the people around him. If he has good friends or mentors, they could help him deal with his emotions in a more positive way.
There's a fanfic called 'Minato's Return'. In this story, Minato somehow returns and gets to see Naruto's growth. It's really interesting to see how he interacts with Kushina, who is also there in the story, and how they both guide Naruto in new ways. The descriptions of their family bond are heartwarming.