Necromancer idea,  could I use a spell like contagion on the city or village water supply to slowly kill the cities inhabitants

I would then go throughout the city and visit the dying to "bless" them before passing, and while they are on their death bed i use finger of death for an easy permanent zombie control

Any other ideas on turning a city into my undead slaves? 

You'd have to have a plan for how to deal with the plucky party of four murderhobos that would for some reason respond to the call to investigate the strange illness afflicting the city!

I would say that you would want to renew the contagion avery few days, that way it doesn't wear off. I would do it in the morning, that way all day people are getting infected and you could pretend to be an early bird, not in the cover of night where you would be a large suspect if anyone saw you.

You don’t need to cast contagion, historically wells like that were spoiled simply by dropping a corpse (human, pig, whatever) into them.

Seconding IamSposta: Infecting a low-tech, low-magic village is easy enough without magic. Contagion could be used for flavor, but would be overkill.

In any serious plague, there is going to be call for hospice care and body disposal, so you could set up shop as a plague doctor and offer cremation services. People would literally flow into your office, never expecting to leave, and might even pay you for the service.

The bigger the city, the less attention you would draw.

That’s a great idea.

aaaand... not to mention the fact that contagion is cast on a person, not a thing like a water source... read the description a bit more closely.

I like the general idea, though...