Which son did Tziporah now circumcise?
Rashi (on Pasuk 24) and Ramban (on Pasuk 20) 1 : Tziporah circumcised Eliezer.
Targum Yonasan: She circumcised Gershom. 2
How did she know why the angel wanted to kill Moshe?
Rashi: Because, in the guise of a snake, it swallowed Moshe, first from the head down to his thighs, then from his feet up to his thighs.
What did Tziporah mean when she said "Ki Chasan Damim Atah li?"
Rashi: She meant that the baby almost brought on the death of her Chasan (Moshe), on account of the B'ris Milah.
Seforno: In defense of Moshe, she was explaining that she circumcised the baby on account of the condition that he had made with her when he married her that they would circumcise their sons and produce the blood of the B'ris.
Targum Onkelos: It is on account of the blood of the B'ris Milah that my Chasan has been given (back) to me. 1
Targum Yonasan: The son-in law (Moshe) wanted to circumcise but the father-in-law (Yisro) stopped him; 2 and now the blood of the B'ris Milah will atone for my Chasan.
Riva: The baby is called a Chasan due to the Mitzvah (lest we say Chasan Bar Mitzvah, Chasan Torah), and my husband's blood is [about to be] spilled due to him!
Da'as Zekenim, Hadar Zekenim (both on verse 26): She thought that Moshe would be killed for blemishing his own Milah, i.e. she was not proper to be his wife. She circumcised the baby, lest it is due to not circumcising the baby.