In fanfiction, the idea of Naruto attacking Kushina is very uncharacteristic as they are mother - son. It could be a very dark or alternate universe scenario. Maybe in a plot where Naruto is mind - controlled or in a completely warped reality where family bonds are reversed.
Naruto would likely be deeply hurt, not just physically but also emotionally. He has always idolized his mother, so an attack from her would be a huge blow to his psyche. His trust in family and those close to him could be severely damaged.
Naruto might react with confusion at first. He has always wanted to be accepted by the villagers, so being attacked is the opposite of what he expected.
Definitely Sasuke. Their bond is such that Sasuke would not stand by and let Naruto destroy Konoha. He has his own reasons to protect the village now. Shikamaru is also a likely candidate. He's intelligent and would quickly figure out that Naruto's actions are wrong. He would use his strategic mind to try and stop Naruto. And Hinata, who loves Naruto deeply, would oppose him in the hopes of bringing him back to his senses.
He may also choose to defend himself without really fighting back. Naruto values Sakura as a friend and teammate, so he would be reluctant to harm her. He might just try to dodge her attacks and keep talking to her, hoping to snap her out of it.
It could be an old enemy who has resurfaced and found a way to turn Naruto against the Leaf. Maybe Orochimaru, who has always been interested in Naruto's power, manipulates him into attacking the village. Orochimaru could use his knowledge of dark jutsu to influence Naruto's mind.
In this fanfiction, perhaps Naruto could be under some sort of mind control. Maybe an enemy has found a way to manipulate him into attacking the Leaf. It could also be a case where Naruto is misled about the true nature of the Leaf Village, believing it has committed some great wrong that warrants his attack.
One possible plot could be that Kakashi is under a genjutsu. In this fanfiction, an enemy ninja might have cast a powerful genjutsu on Kakashi, making him see Naruto as a threat to the village. So, Kakashi attacks Naruto, but Naruto, being the understanding person he is, tries to break the genjutsu instead of fighting back right away.
Perhaps there's a time - travel element involved. Naruto from the future, who has been through some horrible experiences, travels back in time and, in a state of confusion or rage, attacks Kushina. Or it could be that a powerful villain has the ability to manipulate Naruto's emotions and uses that power to make him turn on his mother. This would create a very complex and dark plotline that challenges the normal Naruto narrative.
In this fanfiction scenario, it could be that the zanpakuto has a mind of its own. It takes control of Naruto and forces him to attack Kakashi. The zanpakuto might be from a different dimension and is trying to create chaos in the Naruto world. Kakashi would have to rely on his ninja skills and his knowledge of various jutsu to counter Naruto's attacks and find a way to free Naruto from the zanpakuto's control.