To win a match of King of the Hill, you need gather more points than the opponent teams. You get points by owning hills: for every second you own a hill, you get points. This means the more hills you own, the faster your points go up. You capture hills by standing on them until they turn your color.
Attacking in groups is very important in King of the Hill. It's hard to kill all enemy players and capture the hill in one push by yourself. If there's multiple hills it might be worth to let a hill alone for the other team to grab, as long as your team still has more hills.