Having read up a little on this earlier this morning, I'd say he was a bit of an anomalous serial killer.
Started killing relatively late, without a particular history of violence. Clearly hugely affected by the death of his grandfather. I'm no psychiatrist, but I sense he got off less on the actual act of killing itself, instead gaining particular pleasure (sexual and otherwise) from having a corpse in his company. Not my idea of a productive weekend.
Many of his victims were especially young and their families must have been through torture.
I suppose technology will make it more difficult for people like this to continue for as long as Nilsen did. It's difficult to imagine him getting away with the same MO for anywhere near as long today.
RIP his victims.