A.T. Robertson Commentary John 21:23

"This saying therefore went forth among the brethren, that that disciple should not die: yet Jesus said not unto him, that he should not die; but, If I will that he tarry till I come, what [is that] to thee?" — John 21:23 (ASV)

That that disciple should not die (οτ ο μαθητης εκεινος ουκ αποθνησκε) (present active indicative), because Peter or others misunderstood what Jesus meant as John now carefully explains. He was rebuking Peter's curiosity, not affirming that John would live on till the Master returned. John is anxious to set this matter right.