It's amazing what "Roots" has done to impressionable viewers, perhaps with "Holocaust" as a booster.

The long American tradition of being ashamed of one's lower-class immigrant grandparents has been ruthlessly rejected by the young, who are now anxious to shed their heritage of assimilation and involve themselves like outlaws in Old Country feuds, for which they do research in Ivy League college libraries and on museum work projects.

This, at any rate, is the premise of "Last Embrace," a film that has third-generation Jews running around New York pursuing ancient vendettas like a bunch of mobsters. The hero, a smoothie secret government agent, is so assimilated that he can't look a rabbi straight in the eye, and the heroine is a Princetonian who has mesmerized a scio of the Peabody family, presumably by telling him the same stories about old Granny Eva that understandably put the Jewish hero to sleep.

No social explanation of this phenomenon is given in the film which is taken from a novel by Murray Teigh Bloom. It is tossed in so casually that one realizes only late in the film that the hero, played with extreme nattiness by Roy Scheider, is Jewish. If only honest Sam Levene, who plays a Jewish underground protecting agent, would take him aside and ask him what in the world a Jewish boy was doing with a dumb blonde wife, a ditto thug of a brother-in-law, and a job that exposes him to the bullets of bandits.

The idea seems to have been to add a new dimension to the mystery movie, which had already gone from simple thriller to psychological thriller. This is a psychological ethnic thriller.

In the psychological thriller, the view has to consider craziness as a possible motive. This is regularly abused by filmmakers, who use the fact of senseless crime as an excuse for leaving logic out of their storis, and thus too easily fooling their viewers. The ethnic element only adds to the audience's undfair handicap.

"The Last Embrace" opens in a sanitarium, with the hero going back out into a world that 1) is crazy 2) he sees crazily 3) contains a new type of craziness. A man who worked for a mysterious CIA unit that destroys its own people has just had a mental breakdown, and who is now among people with the new ethnic madness, is not a hero whose word one can take.

The tantalizing thing about "Last Embrace" is that there are so many other characters whose stories would be interesting if told with only two of the three elements used.

As a Simon Wiesenthal type of detective, Levene does not come close to the performance of Laurence Oilivier in "The Boys from Brazil," but he has rakish charm enough to carry an ethnic thriller without the psychological element. Janet Margolin is madly engaging as a wistful-funny-earnest heroine until she is put through those contrived paces, and would have been good in a psychological-ethnic picture, without the criminal thrills. And Christoper Walken only has two minutes as a snooty government madman, but his sequence suggests a good psychological thriller. CAPTION: Picture, JANET MARGOLIN AND ROY SCHNEIDER AS THE TROUBLED LOVERS IN "LAST EMBRACE."