The ending???

So the lead's best friend (who's gf he has had sex with) gives the lead a gun with blanks in to test him? Test him how, if he wants to kill him this means that he had sex with his girlfriend???

And then the lead who has no real reason to do so decides to shoot his best friend (unknowingly with blanks and end up getting killed off screen by his testing best friend) why?

If anyone would pull a gun, certainly it would be the best friend, right?

I can't make any sense of the ending

It's been a long time. I shouldn't have left you, without a dope post to step to