In this week’s episode of “Frontline,” The Killer at Thurston High (Tuesday, January 18; 10 to 11:30 p.m.; Channel 13), producers Michael Kirk, Miri Navasky, and Karen O’Connor report back at exhaustive, mesmerizing, and depressing length from a year’s inquiry into the origins of 15-year-old Kip Kinkel’s killing spree in Springfield, Oregon, in 1998. The usual bromides do not apply. Kip’s father was a retired teacher and tennis coach, apparently beloved by his students, who built his own house, called Shangri-La, in a friendly middle-class neighborhood just outside Eugene. Kip’s mother taught Spanish. Both loved books, languages, travel, and Kip – which didn’t prevent his gunning them down before going the next morning to the high school that had expelled him and opening fire on 27 students, two of whom also died. We hear about the dyslexia, the shoplifting, the therapy and Prozac, the Marilyn Manson music, and the Web-surfing for porn sites. There would even be a trench coat. But we hear as well about the guns – a 9-mm. Glock, a 20-gauge sawed-off shotgun, a semiautomatic Ruger rifle, a 336 Marlin rifle, a Winchester lever-action rifle, two .22-caliber handguns, and thousands of rounds of ammunition.