If Victor's father hadn't written Victor anything but a few lines, how would that have been a cruel kindness?

In context, a few lines would have been a cruel kindness because the mention of William's death would have been omitted. Victor's father would merely have waited for his arrival to apprise him of the tragic news. None-the-less, keeping tht news from Victor would have been a cruel thing to do.

"You have probably waited impatiently for a letter to fix the date of your return to us; and I was at first tempted to write only a few lines, merely mentioning the day on which I should expect you. But that would be a cruel kindness, and I dare not do it. What would be your surprise, my son, when you expected a happy and glad welcome, to behold, on the contrary, tears and wretchedness? And how, Victor, can I relate our misfortune? Absence cannot have rendered you callous to our joys and griefs; and how shall I inflict pain on my long absent son? I wish to prepare you for the woful news, but I know it is impossible; even now your eye skims over the page, to seek the words which are to convey to you the horrible tidings.

"William is dead!—that sweet child, whose smiles delighted and warmed my heart, who was so gentle, yet so gay! Victor, he is murdered!