LOS ANGELES, Jul. 22 – The state says it knows why beautiful actress Sharon Tate and six others were slain last August, and “the motive is even more bizarre than the killings themselves.”

That motive, Deputy Dist. Atty. Vincent Bugliosi said Tuesday, won’t be disclosed until opening arguments and testimony begin Friday in the trial of four persons charged with murder-conspiracy.

The prosecution will put 40 to 50 persons on the stand. But the spotlight will be on pretty, petite Linda Kasabian, 21, who reportedly has promised to tell details of the killings in exchange for her freedom.

Mrs. Kasabian, sandy-haired mother of two infants, is expected to be granted immunity from prosecution after she testifies.

Bugliosi said she would take the stand Monday and was expected to testify for “three, four or five days.” He added, “It will be key testimony.”

Mrs. Kasabian also is charged with murder-conspiracy but has been granted a separate trial. She was a member of the nomadic, hippie-style “family” led by a shaggy-haired, bearded ex-convict, Charles Manson, accused of planning the killings.

Manson, 35, is on trial with three shapely women followers, Susan Atkins, 21, Leslie Van Houten, 20, and Patricia Krenwinkel, 22.

Mrs. Kasabian, who has been under heavy guard in jail since it became known she would testify for the state, is expected to tell how she went with other members of the Manson group to the two homes where the killings occurred.

She is said to have waited outside Miss Tate’s rented tomato red house while the pregnant actress and four visitors were stabbed and shot. The next night, Mrs. Kasabian also reportedly waited outside the home of market owner Leno LaBianca while he and his wife Rosemary were slain.

Paul Fitzgerald, head of a defense team of four lawyers, said they would call about 20 witnesses, many of them members of Manson’s “family.”

“In addition to rebutting the prosecution’s claims of guilt, we may have to rebut them on the defendants’ life style,” Fitzgerald said. “We’ll get into drugs and LSD.”

Court was recessed for two days after six alternate jurors were sworn in Tuesday, The alternates, four men and two women, along with the regular jury panel of seven men and five women, will be locked up in the Ambassador Hotel each night for the trial’s duration — estimated at three to six months.

By LINDA DEUTSCH