Millar could also write out the rape scene since he's already changed Dave and Katie's relationship in the film. Or it could be off camera as you suggest.
Well I don't think it should be written out. The fact that Dave and Katie are an item in the movie only makes it more disturbing and personal. You're really going to want to see Kick-Ass kick The Motherfucker's ass after that.
Besides I've seen Death Wish II
which had a long, prolonged, extremely graphic,
ten minute long gang rape scene. Then a few minutes later a catatonic woman gets raped. Then later on some thugs are trying to abduct and rape a woman. That shit was way more disturbing and graphic than anything Millar wrote.
The escalation between Dave and Chris is one of the interesting things in the second volume. It's definitely going to be interesting to see how the film does...especially with a different director.
It reminds me of the end of Batman Begins
when Gordon brings up the subject of escalation with Batman. Of course, The Motherfucker escalates things to a level that even the Joker would consider crazy.