After seeing the replay, i'm rewriting my post.
We see messi running at the defence, with loads of space on his left, and loads of space behind the defence. Horrible position for the defence. At this point, he will continue to run at the goal, and have a good shooting chance. Normally a cm needs to make a professional faul here, but they were positionned very bad.
One of the defenders had to step out to block off messi. Barzagli steps out, and Messi simply lunges into that space on the left. He then proceeds to have a pretty damn good shot, none of the other cb's block.
messi made his entire name in these situations. on the run, lots of space to move into, drawing out defenders and taking them on speed to the wide area.
Barzagli isnt a "stopper" style of cb. He's good holding his line, positionning and beeing terribly difficult to get past in position. Chiellini isnt as good at that, but is excellent at stepping out, and making sure he'll eighter rocket the ball away, or get some faul that at least stops the play.
Saying Barzagli shouldnt step out is silly. Messi would have a clear shot, you have to block him off, or he'll shoot or send a defender trough, or tries a 1v1 on the edge of the box to draw a faul.
You know, like he did on the winning fk in the supercup just ago