So we all know, that everyone in Death Note has a lifespan and one day, they'll die when their lifespan runs out. But doesn't that mean, that the victims who get killed by a serial killer are destined to being killed by him? Or are their deaths destined the moment a serial killer decides to kill them?
The people can be killed before their lifespan ends using Death Note. The lifespan of a person is decided by fate and doesn't keep on changing, like when a serial killer decides to kill that person, his lifespan changes. No, the lifespan is already decided, so if the lifespan is already short the serial killer succeeds; if it isn't, the serial killer fails. Also, if a Shinigami writes someone's name which in turn increases the lifespan of others, that Shinigami dies. Humans don't have this problem while using Death Note.
There's their regular lifespan which is when they are going to die for whatever reason like serial killer, or bus accident (this reflects in their lifespan). BUT they can be killed before their time using the deathnote, which then sends the the difference in lifespan to the shinigami to extend their life.