What are the connotations of the word "Lehashmido" - as opposed to 'Lehargo'?
Rashi: It implies 'to kill his children'. 1
Rashi writes that Hashem wanted to kill Aharon's children, and Moshe prayed about this. Why did his Tefilah not prevent Nadav and Avihu's death?
Rashi: Moshe's Tefilah helped partially
This is like the Midrash Rabah explains, 'Tefilah As'sah Mechtzah''. See also Sifsei Chachamim.