Simon didn't stop before Chelsea stopped screaming. Simon's anger at Chelsea was not only because of tonight's incident. He has been following Zalina for a long time and witnessed how she bully and denounced Zalina while studying at university.
Simon had been holding back his anger for a long time, and this time he just let it out.
When Chelsea had stopped screaming and just lay weak and helpless, Simon stopped his action and got out of the car. Simon, who at that time was a wolf, turned himself into a human. Simon then left them, either dead or alive.
The next morning Chelsea and Vanessa were found on the side of a road in one of the city park's woods. Their situation was very sad, even though their lives were still saved. It seems like Simon deliberately let them die slowly, or indeed Simon wanted the two girls to suffer physical injuries that caused disabilities.