The actual killing does seem to have been very frenzied. However, it might not have been intended as a murder to begin with. It might have been blackmail or a slightly less than above board business deal. Penny seems to have been stressed from at least the time she was seen driving slowly towards the centre that morning. Once they got to the centre carpark it might have become clear to Penny that this situation was out of her control. Within the close confines of the two front seats of the car, and with Penny fighting for her life, possibly holding the wallpaper samples in an attempt to protect herself. Once the stabbing started it may have been a mixture of panic and anger on the part of the perp as he was unable to strike a fatal blow.
The fact that they went to the drivers side to continue, suggests either uncontrolled frenzy, or a need to finish poor Penny off, as she may have still been showing signs of life. Of course, if there were two people involved, could it be possible that the other person carried out the stabbing from the drivers side. Both then getting into the pre parked car and departing. It would take two identical knives, or the original perp passing their knife to their accomplice. The car park was busy that day, however nobody saw or heard anything out of the ordinary. So the location must have been quite isolated. It's all speculation, but worth considering given how little evidence there actually is.