Genesis 27:41

The days of mourning, &c. Cf. Genesis 50:3-4; Genesis 50:10. The meaning is obvious. Esau says in his heart, "Isaac my father is on the point of death: no sooner shall he die, than I will take revenge. Even while the customary mourning is going on, I will slay Jacob." Before seven days have elapsed (cf. Genesis 50:3) he will have had his revenge. For "say in one's heart," cf. Genesis 8:21; Genesis 24:45 (J).

Very improbable is the interpretation which makes "the days of mourning, &c." mean "the days of mourning by my father," i.e. "for the death of Jacob."

then will I slay The word "then" is simply "and" in the original. The clause is consecutive. There is no adverb defining the point of time. But the idiom emphasizes the dependence of the second clause upon the first.