I'm going to build a robber mage in the game. Here is my build:
http://uk.ign.com/builds/the-elder-scrolls-5-skyrim?d=00000000000000000011111111111111111100011011110000000000000011101111120111101111105111111001000111111000000000000000000000000000000000000000000000510001000000000000000000111010011100000000011111
He has no direct combat skills. He can only do damage to enemies with conjuration & his followers. He has excellent restoration skills & defensive spells. He could power up his followers with the illusion spell "call to arms". He could pick any locks in the game. Sneaking & pickpocket is useless as a robber, anyone in his sights will be killed in few seconds. He could make any best possible equipments with smithing, enchanting & alchemy for his robbery buddies. For the speech skill, he only needs to sell stolen items to merchants. No bribing to guards are needed, guards will be killed easily.

But what will happen if I kill all the npcs in the game? I mean the npcs that are not generic:
http://www.uesp.net/wiki/Skyrim:People

some are essential though

you cant kill all the NPCs because some are needed for important quests however you can kill all the others and you will miss out on other quests

It will be a very empty Skyrim.