Spoiler Below Yeah I agree, I mean, I know killing him in the heat of the moment felt right for them probably, but they'll have a rough time with that one and the cops. Maybe it's to allude to an unhappy ending for EVERYONE, even though the girls were triumphant with him, they still get thrown in prison for aggrivated assault.
The way I understood his sissy-ness at the end was that it was supposed to be ironic. Here we have a cool, collected, suave psychopath, and then BAM, whoa, he's not exactly who you thought he was. Serial killers are glorified a lot in movies as unshakable beings, but I learned in my forensics class that almost all of them (in real life) have some kind of weakness that just makes them break down and the result is either intense rage or absolutely losing it.
I know it was never STATED in the movie that he wanted the upper hand as a motive, but maybe his breaking point was the loss of control in a situation. Yeah, he felt comfortable in his car as he was losing control on the ROAD, but he always had his seatbelt on. He didn't have time to put it back on after the girls shot at him and before they came back, so during the whole chase, he didn't have a seatbelt on.