They wanted to assassinate one famous person for the infamy that it brought. Chapman, sadly, succeeded, and Hinckley, happily, did not (you may disagree on whether the latter's failure was a good thing).

And serial killers are called that because they kill one person at a time (or, rarely, a couple of people at a time) in a series of killings. It's a completely different behavior pattern with a completely different psychological profile. And serial killers are REALLY not affected by gun control, since many of them use knives and other weapons designed to slow the death and allow them to get more of a high off the act of the killing.