``Koch a Killer?'' That's what the tabloids scream after a wedding that New York's feisty mayor has agreed to perform leaves one of the guests, billionaire developer Karl Krieg, stabbed to death by a knife somebody managed to sneak past the metal detectors in City Hall. Since His Honor is the only person who didn't get checked by the detector, he has every reason to turn detective and unmask the party who dideven though the other guests, all of whom were being blackmailed or ruined or abused by Krieg, are such nice people. But then everybody in this pipe dream of mayoral intrigue is basically nice; that's one of the ways you can tell that even though it stars the erstwhile mayor of New York (Citizen Koch, 1992, etc.), living out his fantasy of being mayor forever, it's a work of fiction. Other ways: His Honor vows to present only indisputable facts, not personal opinions, to reporters, who are all happy to see him; all the gentle suspects lie to the mayor during their first chat but tell the truth during their second; Koch smiles at everybody he meets, and they all smile back. How's he doing? Let's just say that being under suspicion of murder does nothing to cramp Koch's bumptious, genially self- approving personal style. Whatever you think of this sporadically entertaining mystery debut, he's bound to love it.