Why so extremely tragic ending?

Because they wanted to impress a viewer / make it more memorable to us. A cheap trick, such tragic, over-done endings, and it also ruined otherwise an OK experience with this movie for me.

"Bangkok Love Story" seems to be based/styled on a manga, where everything is very dramatic and serious. It plays out like an old fashioned tearjerker, and it is relentless in it 's despairing picture of destroyed lives; no one gets out alive basically, and the tragedy is hypnotic as it unfolds on the screen. The old, AIDS-ridden mother, the young son HIV infected by his own stepfather, the loner assassin too mistrustful to let anyone close to him, and the lost regular guy ready to give his life up for something that he considers real. And none of these people have a chance. i loved this dark, apocolyptic vision of a screwed up and hopeless world, and thought the leads were truly fine in their roles. The camerawork and cinematography were brilliant and beautiful, and this is by no means a gay film, because there is just so much more going on than a relationship between two men. The whole idea of this one was about broken people trying to connect and find a way to live, while fixing their their life mistakes. i remember wanting to kill those bastards that beat up the sick boy in the alley as he tries to get the medicine to his mother. much like "Bladerunner." "Bangkok Love Story' portrays such a nihilistic and cold vision of a certain future. a severely underrated film.

There truly seems to be a competition among Asian film-makers to see who can create the most tragic, over-the-top, doomed love story possible. The gay characters share one special moment together and then insurmountable tragedy ensues. I've seen it in several gay-themed Asian love stories. I think the Taiwanese movie "Formula 17" is a rare exception to this.

agreed. this movie had a good start and should have ended much better.

Why a spoiler in the title?