That was my original intention. And you know what? After buying the game on release and playing every day, I have got a grand total of ZERO kills. Yep. Zero.
Killing another player requires a coincidence of circumstances: low energy rival, the right pickup in the right moment, etc etc. It's not only dependant on your skill.
Oh, I have blasted away hundreds of weapons at hundreds of people, but none of them have been a coup de grace. Plus I don't remember having got a single Plasma pickup online, ever.
What did I do? I set up a 1-rival session with a buddy, we both picked Icaras, and went at it. In 25 minutes I went from 0 to 50 kills, and so did him.
AG Assassin might come naturally. But who's to say that the game will be popular enough for me to get it in, say, one year from today?
It's like Bling Brigade. It MIGHT happen by itself, but if you don't get it now, the online lobby could turn into a barren, empty wasteland and your chances of racing other 7 people with the proper skin get slim and rarer by the day. I say no, thanks. Better to set everything up, getting the damn trophy and proceed to enjoy the game.
I'd rather waste 25 minutes, get it together with Double Kill, and then enjoy the online mode naturally. Sounds much better than ruining all your races by devoting yourself to buggering and blasting other racers.