The world's heroes generally leave the Punisher alone, as he keeps his focus on the bad guys, he's not worth the trouble it would take to catch him. When Iron Man was appointed head of S.H.I.E.L.D., however, he could no longer afford to ignore the infamous vigilante. He figured it couldn't possibly be that difficult to take out a single man with no powers, but then, he's never really faced the Punisher.