SPOILERS (Someone should've put Spoilers in the topic title tbh)
Jack wanted to set off the bomb, because he thought he could change everything. He still had lingering doubts about Destiny. He still believed that he could change -- that he could save the lives of everyone who died on Flight 815. Ana Lucia, Libby, Boone, Shannon, and all those nameless people who died.
The nuke is what starts to erase those last lingering doubts.
And I think Sun's development came as a result of the earlier seasons. Sun at the beginning and Sun at the End are practically different characters. In the beginning, Sun wants nothing more than to get away from her husband. That is her reason for getting on that plane. At the end -- she wants nothing more than to find her husband. It's a full swing. In fact, the reason she gets on the Ajira plane is to find her husband.
The two characters that were truly gypped imo were Claire and Sawyer. We never get to see them reunite with Aaron and Clementine respectively.