Antagonist
Antagonist, or villain, in the story is PIECES. As explained in multiple sections of this website, PIECES has been mistreated, hated, and considered as an outcast due to ITS huge and gigantic composition. Therefore, too much of the unnecessary hatred from the people makes PIECES angry and vengeful. One by one, he starts killing people who mistreat or show unnecessary signs of hatred to IT. Killing starts with William, followed by Justine (prosecuted), Henry Clerval, Elizabeth, and Victor (died in the pursuit of PIECES). Due to ITS villain-like actions and behavior, PIECES would be considered as an antagonist.
Satire
An example of satire in Frankenstein is Victor's importance towards beauty and looks in order to figure out the personality and character traits of a person. Throughout the story, Victor thinks that Elizabeth is his angel, or good-luck charm due to her beauty. But, during the Justine's trial, she does not testify in favor of her even though she knew that Justine was not guilty. This is satirical because, despite being a "good-luck charm" of Victor's, Elizabeth did not do much in favor of Justine.