If we supply arms to someone knowing full well what they're going to do with them then, yes, we're responsible. And we have a dark history when it comes to that subject. The CIA did some pretty nasty things during the Cold War to keep Russian influence at bay. You might also look up Operation Ajax (1953).
With Iranian nuclear knowledge it's not just "innocent civilians" at risk, it's a whole country full of innocent civilians. Actually, it's two countries full because sure as hell if a nuke is hand delivered into Israel by some terrorist you can bet Israel will bomb Iran and ask questions later.