During his confrontation with Henry Morgan, Adam fires Henry's single barreled (and therefore single shot) muzzle-loading pistol twice without reloading.
Adam shoots the muzzle loading flintlock pistol he stole twice without the opportunity to reload it.
Adam fires the flintlock pistol from a short distance, hitting remarkably close to the heart region as also evidenced by the entrance holes shown on Henry's clothes during the following moments of the scene. A Wound caused in the heart region, by such a pistol bullet, fired from such a distance would be devastating, resulting in an almost instant death or in a matter of very few seconds. Henry would not have had the time to accomplish what he's seen doing after being shot by Adam.