Any Lời khuyên on how to get a cat to not be mean?

I got a kitten last spring from the animal shelter. She was fairly nice until now, but ahe didn't like being picked up hoặc held. Lately she has started to get really mean, she hissed at a baby for crawling bởi her. She also crawls around the counter taking any thực phẩm off in and trys to steal thực phẩm off your plate. My other cat is overweight so we can't put thực phẩm out all ngày for my kitten. When bạn put thực phẩm out for her, she literally comes running at it and gulps it down like she's afraid someone will take it. My mom hates her and wants to get rid of her, but I'm hoping someone can help.