Comes now Sirhan Sirhan claiming that he is sorry for Robert F. Kennedy's death and that the senator would have wanted to see him released soon. What is the occasion for the assassin's sudden, grossly distasteful effort to exploit his victim's reputation for compassion? The most notorious prisoner at the correctional facility in Soledad, Calif., is asking California's parole authority, the Board of Prison Terms, to let him out in 29 months.

Mr. Sirhan was sentenced to death for murder in 1969. When the California death penalty statute was declared unconstitutional in 1972, this was commuted to life imprisonment. Three years later, the Board of Prison Terms, deciding to treat him as it would any other murderer, set a date for his automatic release on parole. That date, Sept. 1, 1984, is approaching, and the district attorney of Los Angeles has asked the board to reconsider its 1975 decision.

Most murderers, Mr. Sirhan's lawyer argues, serve only seven or eight years, and a stiffer penalty imposed just because the victim was prominent is unjust. This argument, however, glosses over the special facts of this case. Mr. Sirhan's offense was more than a grievous crime against an individual and his family. It was an assault on the political system and was immediately recognized as such: after Sen. Kennedy's death, Secret Service protection was extended to presidential candidates, and the killing of a member of Congress was made a federal crime.

Actually, it is not necessary to reach the question of whether Mr. Sirhan should be held to a stricter standard than a nonpolitical criminal. Nor, given the notorious unreliability of inmate testimony, should much weight be put on a fellow inmate's testimony that Mr. Sirhan plans to kill Sen. Edward Kennedy and join forces with Middle Eastern terrorists if he is released. He has written letters from jail threatening the lives of his lawyer, an author and a prison official. Parole authorities did not know of these letters when they reached their 1975 decision.

But the most persuasive reason for keeping Mr. Sirhan in Soledad is that not until his parole hearing this week did he give any indication of remorse for his crime or any sign that he has changed fundamentally since 1968. Such evidence should be a prerequisite for parole. Simple good behavior should not qualify an inmate for automatic early liberty. His release should be set, not on the basis of some computerized averaging of time served by all murderers, but only after a close analysis of his character, his crime and his potential for good or evil after he is set free.