Board Thread:Questions and Answers/@comment-208.106.84.126-20130104022701/@comment-5588400-20130104234519

Because Gumball murdered Mr. Cuddles' spider family (he was the only one to survive, although Gumball broke one of his appendages.). Since that day, Mr. Cuddles has been training, getting stronger, and planning revenge on Gumball for what he did to his family. Eventually, Penny found him on Gumball's lawn, and saw that Mr. Cuddles' arm was broken, so she took him in and nursed his arm back to health. Time passed and one day Mr. Fitzgerald flushed Mr. Cuddles (or so he thought).

You know the rest of the story, when Gumball found Mr. Cuddles on the toilet, he didn't recognize him (he thought he klled him), and that was when Mr. Cuddles' got his revenge.

They didn't show it in the episode, but after attacking Gumball, Mr. Cuddles went to his family's grave and laid flowers in memory of his deceased love ones.