While North Korea is the prime suspect right now, I wouldn't rule out other possibilities.

After all, Kim Jong-nam's extravagant lifestyle was costing someone a lot of money.  And when there was a chance that Kim Jong-nam could one day be supreme leader of North Korea, there was some benefit to spending some money to curry political favour.

The problem with just cutting Kim Jong-nam off is this.  Kim Jong-nam possible knows a bit too much, ant that bit too much might cause problems if it ever got out in the wild, so to speak.

So if you have the means to kill the man, you can solve two problems with one stone.  After all, the dead can't tell secrets.