I had to rewind it too to see who shot whom. My wife said Dwight was shooting at Simon but Simon ducked and it hit Sara, but after the rewind Simon didn't duck out of the way but he was walking up to Tara and was going to kill her (why not shoot her from afar, who knows?!). Dwight shot the crossbow to the side and past Simon to hit Tara. I'm still not sure why Dwight cares about Tara. But it appeared to me that he shot her with the crossbow before Simon got a chance to shoot her with the gun. Confusing and dumb scene, yes.
I guess the whole point of the scene was to show the audience that Daryl saw Dwight shoot Tara, thus the conflict between who's side Dwight is on. At this point, I couldn't care less anymore. First Tara wants Dwight dead and Daryl doesn't. Then the complete opposite.

I wish Morgan would kill Tara and Daryl on his way out west.