What did Yakov have in mind when he said that whomever Lavan's gods will be found with him, he will die?
Rashi, Targum Onkelos and Targum Yonasan: He meant literally that whoever stole them would die prematurely; indeed, as a result, Rachel died on the way. 1
Seforno: He thought that it was one of the slaves who had stolen them in order to worship them, in which case he deserved to die.
Rashbam: He did not mean literally, that he would die, but that he personally would punish the culprit - bodily.
Ohr ha'Chayim (31): Lavan's claim that he stole the Terafim lest they tell that he fled was plausible (Yakov admitted that he fled). To counter it, he swore with a curse, like one who imposes Cherem amidst Safek.
Riva: Even though Lavan did not find them, the curse of a Chacham is fulfilled, even if it was conditional and the Tenai was not fulfilled (Makos 11a). Moshav Zekenim asks why Tzadikim, Tana'im and Amora'im cursed on Tenai. He answers that it is fulfilled unconditionally only if it was not possible to fulfill it, like here.
Why is it important to know that Yakov was unaware of the fact that Rachel took the Terafim?
Seforno: To insinuate that, had he known, he would neither have had the audacity to deny Lavan's claim nor would he have cursed the person who took them.
Why did Yakov need to say that whoever stole the gods will die? A Ben Noach is killed for any of his Aveiros, including theft!
Ohr ha'Chayim: He will not have a claim that Lavan gave them to him, or he took them amidst forgetting.
Ha'Emek Davar: Yakov was not so sure that none of his slaves took something else from Lavan, and he did not decree death for this, only for idols. One who intends to serve idolatry in Yakov's house should not live!