Moby-Dick Questions Chs. 123-124: In ch. 123, “The Musket,” how does Starbuck justify

In ch. 123, “The Musket,” how does Starbuck justify murdering Ahab?  How do you interpret Starbuck’s line that Ahab would be “the wilful murderer of thirty men and more, if this ship come to any deadly harm”? (p. 558 [387])  Why doesn’t Starbuck kill Ahab?  The phrase “wrestling with an angel” at the chapter’s end is a biblical allusion to Jacob wrestling with an angel; what light does it shine on Starbuck’s dilemma?  How does Ahab’s work with the needle relate to other metaphors of threads?  Why does Starbuck look away as Ahab “fixes” the needle?