Who killed the antiques dealer

The same person who killed Todd Johnson.

The antiques dealer, Montero, grew up in the same hometown as Todd Johnson. Montero's job wasn't just an innocent antique dealer. If you remember, he was actually running guns in exchange for rare pieces of art/antiques. One of the guns he "ran" was the same gun used by Johnson to murder cops.

The Red John mole, one of the five people J.J. LaRoche suspected of setting Johnson on fire in episode 3x09, was trying to "tie up" loose ends while also trying to frame one of the other four LaRoche suspects . In this instance, the mole was trying to frame Hightower, knowing she had a previous 'intimate' relationship with one of the cops Johnson killed... thereby giving her motive to kill Johnson, and her fingerprint was found on the bone used in the crime at the museum.

The Red John mole, of course, turned out to be Craig O'Laughlin.