When Mr. Cat suggests attacking Pretty with a chainsaw, Quack Quack, despite being the nicest member of the main four, immediately agrees. There are two possible reasons:
Pretty made fun of not only Quack Quack, but also of his beloved friends (the rest of the main four), and his girlfriend, Eugly, so he was mad at her for hurting the people he loves.
Mr. Cat always attacks Quack Quack with his chainsaw, and Quack Quack thought it would be nice to see this treatment applied to someone who actually deserved it.
The main four basically own the court system and can set up a Kangaroo Court to punish anyone they dislike.
The photo Pretty takes of her kissing Mr. Cat depicts Mr. Cat lying on the ground with his eyes closed, surrounded by empty bottles. It's possible that Mr. Cat had passed out from drinking too much and Pretty took advantage of his unconscious state.