As a general rule, assassination is not an act to be condoned. That being said, one might argue that in terms of history, JFK and MLK "live on" because they did not live on. If JFK had lived a normal life span he could well have ended up looking to history as William Jefferson Clinton does now, and if MLK had lived a normal life span he could well have ended up looking to history as Jesse Jackson or Bill Cosby do now.
It is the same principle in music. Some rock and roll acts sell way more albums after they overdose or get killed in a plane crash then they did when they were alive.