She knows but she appears to be dealing with it. For all we know reimu and Marisa got attacked by a raper alice but didn't kill here before she could start trying to rape reimu.
When people upload they don't purposely edit the image and cut anything off. This is how it was presented at Futaba Channel. Furthermore, this artist has a history of abrupt endings.
Also, the small Alice jumping at the foreground adds an extra degree of misery, by showing a yukkuri jumping for joy in your face and contrasting with the dead stare of the Marisa. This image succeeds in stirring up emotions.