How does kindness beget cruelty?
(25,19) “These are the generations of Yitzchok the son of Avrohom; Avrohom begot Yitzchok.”
It is well known that the Patriarchs (the Avos) and the Matriarchs allude respectfully to the soul and the body. That is, the Avos are the root of the holiness of the souls of Yisrael. And only three people are called the Avos, because each one drew down upon himself and upon his offspring after him a new holiness; Avrohom drew down the holiness of Chesed (lovingkindness) from the upper realms, Yitzchok drew down the attribute of Gevurah (might), and Ya’akov drew down the attribute of Emes (truth), as is well known.
And we are obligated to serve Hashem with each one of these holinesses. Thus, it is necessary to do Chesed to oneself and to everyone else, as it is written in Mishlei 11:17 “A kind man does good to himself”. But serving Hashem also includes that which Rava said in the gemora Eruvin 22a “The Torah endures only in someone who becomes cruel concerning his children like a raven” (one who learns Torah at the expense of providing comfortably for his children), and this is from the attribute of Gevurah. But this is astonishing - how is there a place for cruelty like this?
However, on the contrary, this cruelty stems from the Chesed that one is required to do with one’s soul, to serve Hashem constantly. This is what gives rise to this cruelty. And this is the allusion of the posuk “and Avrohom begot Yitzchok” - Chesed gives rise to Gevurah. And it is also necessary to speak truth in one’s heart, and similarly in all the other traits, and it all comes through his being cruel upon his his children, as we have said.
Now, the mitzvos are called "these", as it is written “these are the mitzvos”. This is the allusion of our posuk: “These” - the mitzvos, “are the generations of Yitzchok”. That is, through cruelty and struggle it is possible to fulfil the mitzvos. And these attributes of might and cruelty are the “son of Avrohom”, coming from the attribute of Chesed. And the posuk continues to explain that “Avrohom begot Yizchok”, like I wrote, that the attribute of cruelty was born from and came from the Chesed that a person is obligated to do for his soul.