Spoilers to come, I am 90 percent sure it's British, I think between 1960 and 1990. Color. Two teenagers are friends. I picture a rural or small town, not a city. They hang out and there may be scenes in one girl's house. There is something that the mother of one of the girls does that results in the girls killing her. They lure her out of the house and are walking a path in an outside area, one behind the other. The mother is pleased that they have shown an interest in pleasing in some way. One of the girls picks up a rock and hits the mother on the head. She doesn't go down soundlessly but it's pretty gruesome.
Thanks in advance!