The grey squirrel is generally considered to be less of a problem than red quirrels. Red quirrels get into wires underneath your car and chew them, they get into garages and destoy stored material. Grey sqirrels, on their own, rairely cause any known problems to man. I don't know if they kill red squirrrels, but if they do, its just a natural responce, probablly better left untouched by man.
If you feel like you have a HUGE population of squirrels around you, check to see if a neighbor happens to be feeding them! :)