Towards the end of parshat Emor, we hear the case of the blasphemer.
The midrash also states that וַיִּפֶן כֹּה וָכֹה, וַיַּרְא כִּי אֵין אִישׁ - "And he looked this way and that way, and when he saw that there was no man" - means that Moshe looked with ruach haKodesh and saw that there would be no progeny from this man who would have converted to Judaism.
These two midrashim seems contradictory, since here we have Hebrew progeny of the Egyptian. I addressed this question in the past, in more detail, in a post on parshat Shemot. It is possible they are contradictory midrashim.
However, the most likely answer can be seen from an analysis of Bereishit 4:9-10:
Demai, blood, is plural, and the Midrash takes off on this to say that he killed not only his brother but every descendant that his brother was to have, who was no prevented from being born. From here we learn that anyone who kills an individual is as if he destroyed the entire world (for descendants of such an early person would have been many) and anyone who saves the life of an individual is as if he saved the entire world.
This seems the motivation behind the Midrash on וַיִּפֶן כֹּה וָכֹה, וַיַּרְא כִּי אֵין אִישׁ, which focuses on worthy descendants to come. It is not the merit of those descendants (of which the blasphemer perhaps anyway does not possess much) but rather the preventing of their being born. Since this event with Moshe happened after the Egyptian slept with the Hebrew's wife, she was already pregnant or on her way to becoming so. The killing of the Egyptian taskmaster would have no effect on eventual progeny, and this was what Moshe was looking for.