What does Buttercup look like and why does Katniss say she should've drowned him when she got the chance?

I think this happens in Mockingjay, the third book. Buttercup is a mangy old cat which Prim loves. Katniss doesn't get along with Buttercup and thinks having a pet is the last thing they need to worry about,

Scrawny kitten, belly swollen with worms, crawling with fleas. The last thing I needed was another mouth to feed.

Scrawny kitten, belly swollen with worms, crawling with fleas. The last thing I needed was another mouth to feed.