[Whatever Belloq did, whatever his reasons for leaving Niko behind, Boone was not expecting it to be because he was a traitor. He'd just assumed Belloq got scared and fled, leaving Niko behind and assuming he was dead. This... this is different.]

He told everyone you were dead and that he was sorry. Said you got overwhelmed by skeletons, and he ran. He locked you in there on purpose?

[Why would he do that? What could he gain from killing Niko?]