IMO,Jack was murdered,bearing in mind the environment in which both of them and their relationship had developed.Of course,story is deliberately ambiguous,or better said,open enough as that everyone of us has their own opinion,and surely everyone is real and good...-remember the case of the "Jack,I swear..."-,In this sense,it could be seen as a result of Ennis paranoia;but we can't forget that Jack in the last times had lowered his guard a lot...It's tragically curious and ironic to see as Ennis had spent half his life protecting themselves against this possibility,up to the point of a constant self-denial,to arrive at the end at the same result he had been trying to avoid...or,at least,with a lot of possibilities of being so.
What's more,I tend to believe that Lureen knew the truth,she was involved in the tragic result;her way of explaining what had happenned seems to cold for a woman who has just lost her husband,the man is supposed she loved.Her father hated Jack and probably she did too,because he was at the point of leaving her-we don't know if for being with Randall-and during all their living together,she surely guessed his tendencies.There's nothing worst that a person who feels deceived and humiliated to turn suddenly all this love into hate.She only cried a little,then,when she guessed-or better said,she knew-who was really Ennis and that BBM existed and was her husband and him "love nest".I think it's clear.
And it was easy,because as I said before,Jack was more reckless than ever in the exhibition of his same sex companies;surely because,as Lureen said,he was drinking a lot and thinking even in death-his wish to have his ashes in BBM...-.Because he didn't have in same way a reason for living,the idea of a "sweet life together" has been broken into pieces;for this,he was bolder because he didn't mind what could happen.And in this sense,I dare to say that Ennis was the "intellectual",or "moral" author of his death,because he had killed long time ago the Jack he had met,the lively,glad and dreamer Jack,to turn him into a shadow-and not only for year's passing,it's sure...-.