BBM is (among many other things!) a cautionary tale about what happens when love is hidden or denied. It tells the story of the unresolved love between Ennis and Jack, their struggles and regrets with the road not taken, and about the pain they cause for themselves
and others in denying who they are. It's about the choices they have made that maybe say "yes" to one person, while at the same time, saying "no" to another...
After that awful scene with Alma in the kitchen after Thanksgiving dinner at her (new) place, I think that it was only then, that Ennis finally discovers the true extent of the damage and pain his long and secret relationship with Jack had actually caused Alma. Alma had been silent all those years about her feelings. Apparently, she had never mentioned any of this to him before...not even during their divorce...!) She had never ever even told him that little story about her note on his fishing line until that night...
I think that Ennis must have been sooooo in complete and utter
shock after hearing all that she had had to say in that dramatic outpour of hers. I truly think that he didn't even have the slightest inkling that Alma "knew/or even suspected" anything of what he had with Jack. (Just in that particular way, I think that Ennis had always been pitifully "innocent".) Right up until that unexpected confrontation in front of the sink, where Alma suddenly releases all her pent up bitterness towards him, it had remained unbeknownst territory to him. It all seemed to fall on his shoulders that night simply "out-of-the-blue", and it surely caught him off his guard. It must have also been absolutely DEVASTATING for him to hear her call his lover: "Jack
NASTY!".
I don't think that it was ever his intention to (purposely) hurt Alma. I think that he truly always meant to build a real and solid life with her. Life just dealt him out a bad hand of cards, and so it just didn't work out for him with Alma...
To have heard all she had to say that night must have immediately caused him so much pain and sorrow. He must have felt sooooooooo guilty for having caused her so much suffering, that he instantly leaves and actually
WANTS someone to beat him up and cause HIM some pain in return for his past deeds against Alma. I know that this sounds
twisted...but a man in pain...with a heart full of hurt and regret, often goes out and does strange things... And, if you ask me, I think he left the house that night, after the fight, in violent emotional turmoil, filled to the brim with anger, rage, disgust, (at himself?) and sorrow (for Alma?). Clearly intending to get either (wildly drunk)
OR to get involved in a fight...
ANY fight! Just to let out all his pent up emotions from that disastrous end of the evening...! (Alma: "Get out! Get out! You hear me, Ennis Del Mar? GET OUT!")
It was an unavoidable "explosion"....

sam