why did the creature want justine to suffer and how did he decide to accomplish that?

Seeking a hiding place, he enters a nearby barn and finds Justine sleeping within. Her beauty, too, both transports him to ecstasy and fills him with bitter despair, since he will never know the pleasures of love. Suddenly terrified that she will awake and denounce him as a murderer, he places the portrait of Caroline in Justine's dress: she, not he, will suffer punishment for the murder. In his madness, the creature thinks that it is the inaccessible beauty of people like Justine that caused him to kill William; it is thus only fair that she should atone for the crime.