what is the name of this movie?

i saw a vintage thriller hollywood movie of 70's in which, a group of youngsters saw a beatiing & murder of a young man in a jungle by men of a small community living in the jungle, the main murderer who kills the young guy, a lover of his wife, is the owner of small gas-station in that community. while murder, the men puts a boiling tar on the victim for fun. they throws the body inside a cave. the youngsters informs the murder to sheriff but he ignores as he is friendly to the community. the murderer, who also leader of community takes some of the youngers as hostage. at the end the murderer/communuty leader is killed by same way by a hot tar on his body & blowing his gas station.