Any advice 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 or held. Lately she has started to get really mean, she hissed at a baby for crawling by her. She also crawls around the counter taking any food off in and trys to steal food off your plate. My other cat is overweight so we can't put food out all day for my kitten. When you put food 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.