Why was Jack Twist Killed?

Rating: 10/10 (28 December 2006)

First Off I would like to say that Brokeback Mountain is simply the best love story that I have ever seen. At first I saw the movie because of all the hype surrounding it. But when I actually saw the movie I never once thought of this being a movie about gay cowboys. This movie was about how strong the bonds of love are. Anyways! I have a question. Could someone please help me before I lose my mind! Why was Jack Twist Killed?!?!? Was it because Lureen found about Ennis? Was it because Lureen's family found out he was going to leave Lureen? Was it because Lureen found out Jack was leaving her and moving to his dad's ranch with some guy? Oh and at the very end of the movie when Ennis looks at his and jack's shirts and says "jack I swear" what does that mean????