He was engaged with Shikandin. Relying on him as your help, you did bring about the overthrow of Shantanu's son. Of you had died without having expiated your sin, you would then have fallen, forsooth, into Hell on account of that sinful deed of yours. Even this which you have got from your son is the expiation of that sin!
शिखण्डिना तु संयुक्तस्तमाश्रित्य हतस्त्वया। तस्य शान्तिमकृत्वा त्वं त्यजेथा यदि जीवितम्॥ कर्मणा तेन पापेन पतेथा निरये ध्रुवम्। एषा तु विहिता शान्तिः पुत्राद् यां प्राप्तवानसि।