Let me explainThe reason is quite simple let's just assume Kawaki actually killed Naruto .
In this situation the first thing Sasuke would do is to go and Fight Kawaki but if Kawaki could defeat Naruto he should be able to that to Sasuke too but in this situation Sasuke might escape using his rinnegan to revive Naruto through his rinne-rebirth and tell him to become perfect Sage by taking his rinnegan but since he will be exhausted he will die and Naruto then will go to fight Kawaki.
In this situation the first thing Sasuke would do is to go and Fight Kawaki but if Kawaki could defeat Naruto he should be able to that to Sasuke too but in this situation Sasuke might escape using his rinnegan to revive Naruto through his rinne-rebirth and tell him to become perfect Sage by taking his rinnegan but since he will be exhausted he will die and Naruto then will go to fight Kawaki.
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