An extremely mean or inconsiderate person who takes advantage of others or behaves in a cruel way (In Venezuela, calling someone a rat is a standard way to label them a jerk or a scumbag).
Dude, que rata you are, don't you see this is a hospital and you shouldn't smoke! / Man, que rata, you went too far dude! How could you tie his shoes like that?
To harass, mistreat, or pick on someone, essentially synonymous with bullying or messing with someone. (The term is derived from a derogatory reference to Jewish people, though it is used colloquially in Argentina to mean general harassment).