And what did she mean when she then declared "Chasan Damim la'Mulos"?
Rashi: When, following the B'ris Milah, the angel let go of Moshe age, she realized that it was on account of the Milah that he was almost killed. 1
Targum Onkelos: Were it not for the blood of the B'ris Milah, her Chasan would have been sentenced to death.
Seforno: She again referred to the condition that Moshe had made with her when they married, but added Peri'ah (the folding over of the foreskin) to the condition.
Rashbam and Targum Yonasan: She was praising the Mitzvah of B'ris Milah, which saved her Chasan from the Angel of Death.
Riva: Initially, she feared that Moshe would die for negligence, and it will not help to circumcise. Now she realized that Moshe was in danger because Milah was not done, and now that it was done, he was saved.
Da'as Zekenim, Hadar Zekenim: Now she realized that Moshe was in danger not due to her, rather, due to not circumcising the baby. (Refer to 4:25:3:6.)
See Sifsei Chachamim.
Why does the Torah not write "Re'ach Nicho'ach" by the goat of the Nasi?