Buttercup does not like Katniss because Katniss does not like him. She sees him as one more mouth the feed, and tried to drown him when he was a kitten.
Buttercup is Prim's cat.
Katniss refers to Buttercup as the “world’s ugliest cat” because he is missing half an ear and a mashed in nose. Katniss explains that she tried to drown him when he was little because she thought he would never survive.
Scrawny kitten, belly swollen with worms, crawling with fleas. The last thing I needed was another mouth to feed. (ch 1, p. 3)
Apparently, the cat holds a grudge. Katniss kept him after Prim begged her, and their mother got him healthy. Katniss is also pleased that he is a good mouser. As a result, she occasionally gives him some entrails and he has stopped hissing at her.
Katniss is always the practical one. She is not hard-hearted. She just doesn't want someone else to take care of, because she already has to take care of her sister and her mother. Yet when Buttercup is practical, and her sister likes him, she decides to keep him.
Buttercup, Prim’s cat who is loyal to her, hates Katniss because she once tried to kill him by drowning. Katniss also thinks the cat is just an extra mouth to feed. Prim names the cat Buttercup because of the cat’s color; she believes the cat resembles the flower. Katniss, on the other hand, believes the cat is very ugly, at some point stating Buttercup is the “world’s ugliest cat”. Buttercup always hisses at Katniss whenever she is around to show his disdain towards her. This goes on until Katniss recognizes that the cat comes with some benefits such as being a great mouser and some friendship develops, as seen when Katniss gives the cat entrails from the animals she brings home after hunting and the cat eventually stops hissing at her.