^the fact that she kidnapped your friends and asks you to kill them ruined the choice imo.
it would have been much better if she didn't have you kill the GF and you just decided "you know what, i think i will be a god and stick with citra" and then you turn around and she kills you. sucks to be you,
as it is, it was way too obvious that you should not choose her - you killed everyone, freed the rakyat from hoyt and rescued your friends. there really is no reason to stay.
I felt no guilt about offing the 'girlfriend', though felt I should have seen Citra's final reaction coming.
Just out of interest, what happens if you save the friends instead?
Dennis gets mad and tries to stab you, but Citra jumps in the way to protect you and ends up dying. Her last words to Jason are "I love you. Please don't leave me." (something like that). Jason says sorry and she dies. Dennis is sad and cries. Jason leaves with the rest of his friends on the boat. Jason has a monologue where he believes hes become a monster and has to find the good in him to be normal again.
When they kidnap Jason's friends and murdered Dr Earnhardt, in my mind, that's pretty much a screw you from Citra and the Ratyat to Jason. I was actually expecting the ending of the game to be Jason massacring Citra and her followers for double crossing him but then they gave you a choice to save your friends or kill Jason's girlfriend (I didn't catch why) just to stay with Citra.
I'm one of the few people who don't hate the protagonist and his friends just because they're privileged rich kids. They're not the ideal characters I would choose but I never hated them so much that I felt they deserved to die. So... the ending was a easy decision for me play through 1.