The motivation of rape is not sexual gratification. Rapists aren't motivated by orgasm, they're motivated by power, anger, low self esteem, etc. Rapists don't even generally like rough sex.

Taki is what would probably be called an "anger induced" rapist. He sees rape as the ultimate act of violation, and thereby the ultimate way to vent his anger, his hate, towards Shuuichi. He saw this assualt as a way to overpower, to dominate, to destroy Shuuichi. He was not lusting for Shuuichi, he saw the rape as an effective weapon in injuring and dominating Shuuichi.

Taki does seem to exhibit the traits of a potential rapist early on: he's self absorbed and unconcerned with others, he's prone to boughts of anger, he attempts to dominate others, he's never anywhere without his 'pack' (ASK), he's broody and vengeful, he's obsessive.

Selfishness and lack of empathy are important traits in a sex offender. In order to get to the mindset of an offender, one needs to be chiefly concerned with one's self, and the pain of others simply can't be a concern. Taki is selfish in spades -- hell, he's a prima donna. He has no sympathy at all for the pain of others, it doesn't seem to be a part of his mental makeup.

Violent emotional reactions are exhibited often in Taki, he's constantly jealous and he always needs to be part of the limelight. If he's awake, he's gotta be the center of attention, people must be in awe of him. He craves stardom, he's obsessed with it. It's his goal in life, and anyone who gets in his way is going to be taken down.

When he's being moody, the other members of ASK clam up, because he dominates them so throughly. However, he doesn't seem to travel around without them very often -- my standing theory is, that he needs the constant approval and admiration of his friends, his pack. If he does anything, it's because someone is watching. That would qualify as "Pack Mentality", where a person is capable of doing many awful things because there's a false sense of security. The whole world boils down to the pack, and the small dynamics of the pack. One could argue that's why Taki hired those goons to help him out -- alone, he had too much sense not to do it, he needed a sort of 'false pack' to get the job done.

All these traits, by themselves, only really describe a bitch. However, his irrational hatred of Shuuichi is enough to push him over the edge, from simply being a bitch and a prima donna, into the realm of a sex offender.