You know, since all the negatives about this scene have been covered already and will continue to be covered, I'm going to change my tune just a bit and detail two things I felt the developers did right with it. I'm not defending Johnny being OOC and I'm not trying to change anyone's opinion; I just want to talk about a few subtle touches that indicate, to me, that more thought was put into the scenario than might first appear.
1. As has been mentioned already, it's not until Johnny calls Trevor a "motherf*cker" that he even acknowledges him, and in doing so, pretty much decides his fate. It's an insult tossed around very casually, and the significance of it as a trigger for Trevor doesn't become apparent until later, so it's something you'll only notice if you watch the scene more than once. I kind of wonder if Johnny would've lived if he simply used another insult, although given Trevor's temper, that's questionable.
2. It's something easy to overlook in the heat of the moment, but Trevor wasn't intending to kill Johnny.
Now, he obviously didn't care that he did, especially since he went out of his way to rub it in the faces of Terry and Clay, and it's entirely possible that Trevor would've killed him deliberately when he took on the Lost in the future, but the scene makes it clear his death was accidental and a result of what was more or less a blind rage. Trevor expects Johnny to still be alive ("Next time don't get in my f*cking face!") and there's a sense of simple frustration in some of his lines ("Get up! GET UP! f*ck you, then!"), as well as a kind of "Drop the act!" disbelief.
Now, I'm not sure what this means necessarily (and like I said, I'm not trying to sway anyone's opinion or "defend" a particular character), but the more times I see the cutscene, the more I'm convinced there are multiple layers to it that many viewers would lose in the initial shock and sudden brutality.