Skynet has a specific origin story, explained in the movie, Terminator 2: Judgement Day.

  In three years, Cyberdyne will become the
  largest supplier of military computer 
  systems. All stealth bombers are upgraded 
  with Cyberdyne computers, becoming fully 
  unmanned. Afterwards, they fly with a 
  perfect operational record. The Skynet 
  Funding Bill is passed. The system goes 
  online August 4th, 1997. Human decisions 
  are removed from strategic defense. Skynet 
  begins to learn at a geometric rate. It 
  becomes self-aware at 2:14 a.m. Eastern 
  time, August 29th. In a panic, they try 
  to pull the plug. Skynet fights back. It 
  launches its missiles against the targets 
  in Russia. Skynet knows that the Russian 
  counterattack will eliminate its enemies 
  over here.

It was an out-growth of the cold war arms race. Nuclear deterrents aren't as much about protection as they are about posturing. Lives aren't quite "saved" by nuclear arsenals.