I personally play a ninja ghost.... I'll kill ones I see no way around or are getting too close to detecting me.
Most used powers/items:
Dark Vision 2
Bend Time 1
Shadow Kill 1
I'm not super far in yet but I'm planning on boosting my level 1 powers to level 2 and finishing upgrading my crossbow.
With a clever use of time stop, possession and knock out darts I've managed to get as far as beating the Golden Cat level with 0 kills. Still haven't gotten a full ghost mission yet though (go without being detected)
And I've also tried doing the nonlethal versions of dealing with the main targets.
My first 2 missions I was killing everyone and testing all the powers but now I start over where I will be super sneaky :3