Fair enough.
I'll skip to the part where we're in the rental car I got at the airport, i'm driving and you're in the passenger seat. We met somewhere public and started driving. We go into town (you know the area and have stated before you've cruised around looking for potential 'prey') and pick up a nice young lad and whore to your liking, preferably at night with little people around to identify the car, time they were picked up, etc etc. If I tell this whole scenario in the exact detail of how it would be needed to be done I'd be here all day, so, I'll skip the guidelines and formalities and cut to the good stuff you want to hear.
We'd drive them to where the privacy is needed and begin. I'll have a gun for the sole purpose of being able to control them easier, but it wouldn't be used. It doesn't have to be loaded if that would worry you. We'd have to play outside the car (again, I won't list the reasons why). What happens next is more your fantasy than mine. You mentioned you'd like a rape to happen, so how hard, in what hole, for how long, in what manner is completely up to you. I'd make sure they'd oblige.
During the 'rape' I think we should give the boy a knife. This way you can tell him to cut her when you'd like, play with her with it, you know, things of that nature. I think it'd be fun to kill mine as he was cumming (I'll take the boy since your preference is the other). I imagine her on her back looking at him when I rip the knife through his jugular from ear to ear. The arterial blood spatter would drench her front, and I don't know which face would be more interesting to look at, his or hers. Kodak moment? There's going to be a lot of screaming next, so, try to shut that up quickly or i'll just punch her in the throat. Screams never really did anything for me except make my ears ring and cause a lot of annoyance.
So as you can see i'm fairly simple. I'll leave my story here so you can post your part on what you would do with the broad. I know I was being modest in the way I described taking out my garbage, but the real fun for me doesn't start until we've finished. Feel free to change what i've said to tweak your fantasy. There's not much I'm not game for.