Why do abductors want somebody dead (even if the victim(s) was / were chosen randomly)?

There are some people who don't have any enemies in life, but soon one person if found after missing for days murdered and left for dead. Lisa Ziegert, an elementary schoolteacher in Agawam, Machusetts was abducted and murdered in 1992. Are there any possibilities at solving the case? Was she chosen randomly? There are many criminals out in this world.